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Is the blastocyst a coeloblastula?

Is the blastocyst a coeloblastula?



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A coeloblastula is that type of blastula which has a fluid-filled cavity. Since the blastocyst of mammals has a fluid-filled cavity too, can it be called a coeloblastula? My book however mentions the two as different, and it gives the frog as an example of animals having the coeloblastula and mammals as an example of having the blastocyst.


This is a bit tricky.

The blastocoel in e.g. amphibians separates ectoderm from endoderm and prevents cells of the blastocoel roof from inductive influence of vegetal cells. If you cut out the marginal zone and squish the roof onto vegetal cells, you will end up with mesoderm only:

Blastocoel is not homologous to blastocystic cavity. One could consider a tiny space between epiblast and hypoblast during gastrulation as a homologue of blastocoel, this however does not appear in mice. Blastocystic cavity forms at 32-cell stage thanks to Na+/K+ pumps as well as aquaporins. The inner cell mass is a mixture of cells expressing Gata6 and Nanog, which will form hypoblast and epiblast respectively.

But worry not, many people still call that cavity a blastocoel, in fact, I've seen it happen on Wikipedia. Anyway, I'd still recommend using separate terms for coeloblastula and blastocyst due to the non-homologous nature of blastocystic cavity and the blastocoel.

My sources:

  • https://kjcscientific.wordpress.com/project-excalibur/
  • Gilbert's Developmental Biology
  • Ćwiczenia z Biologii Rozwoju Zwierząt [Marek Maleszewski et al., University of Warsaw, Polish language publication]

Is the blastocyst a coeloblastula? - Biology

The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage. Cleavage is illustrated in (Figure 1a). After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula, shown in Figure 1b. During cleavage, the cells divide without an increase in mass that is, one large single-celled zygote divides into multiple smaller cells. Each cell within the blastula is called a blastomere.

Figure 1 (a) During cleavage, the zygote rapidly divides into multiple cells without increasing in size. (b) The cells rearrange themselves to form a hollow ball with a fluid-filled or yolk-filled cavity called the blastula.

Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.

In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast and this mass of cells will go on to form the embryo.

Figure 2. The rearrangement of the cells in the mammalian blastula to two layers—the inner cell mass and the trophoblast—results in the formation of the blastocyst.

At this stage of development, illustrated in Figure 2 the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism. The trophoblast will contribute to the placenta and nourish the embryo.


General Properties of Embryonic Development NEET Notes | EduRev

GENERAL PROPERTIES OF EMBRYONIC DEVELOPMENT
1. Morula :- As a result of segmentation or cleavage activities, unicellular zygote changes into a solid ball like multi-cellular structure. In the later stage of cleavage, clusters of sticky, cohering, protruding(outside) blastomeres are produced, which look like mulberry. This stage is termed as morula stage.

2. Blastulation :- Cleavage continues in solid ball like morula and new formed blastomeres start rearranging themselves. Cell - aggregation starts in blastomeres, due to the movement of these blastomeres a cavity appears in the embryo, it is called blastocoel. This cavity is a schizogenous cavity in origin i.e. it is formed by the separation of cells. Cell aggregation is also known as cohesion. Blastomeres arrange themselves in the form of a layer around the blastocoel, this layer is termed as blastoderm. The embryonic stage is now called a blastula, and its formative activities are called blastulation.

Types of blastula
The shape of blastula depends on so many factors e.g. size of eggs, amount of yolk, distribution of yolk in the eggs, frequency of cleavage, and number of cleavage divisions. According to these factors, we can classify blastula of different animals in different categories.

(a) Coeloblastula
(b) Stereoblastula / solidblastula
(c) Discoblastula
(d) Blastocyst
(e) Superficial blastula or Periblastula

(a) Coeloblastula :- Blastocoel is wide and clear in this blastulation, it is completely surrounded by blastomeres on all the sides i.e., blastocoel cavity is situated totally inside the embryo. Blastomeres are very small in size as compared to blastocoel.

Example - Eggs of Amphioxus, Coelenterata, amphibia, sponges etc.

Coeloblastula of amphibians is called amphiblastula because in it blastocoel cavity is accentric in position and more towards the animal pole (amount of yolk is more towards the vegetal pole). Amphiblastula of sponges is a free swimming larva, its blastomeres are flagillated. This larva swims freely with the help of these flagella. It is a unique feature in sponges.

(b) Stereoblastula :- In this blastula, blastocoel is very narrow or obliterated. Blastomeres are large as compared to blastocoel.

Example - Eggs of Neries, Phylum Mollusca.

(c) Discoblastula :- It is found in those animals which have discoidal eggs. There is a cavity present in between blastomeres and yolk, it is called sub-germinal cavity. It is a type of cavity, which is surrounded by yolk on one side and by blastomeres on the other side.

Examples :- eggs of reptiles, birds and prototheria.

(d) Superficial blastula or periblastula - In centrolecithal eggs, cleavage occurs only in peripheral region. The layer of blastomeres surrounds the centrally situated yolk. Blastocoel is absent in this type of blastula.

Example - Eggs of insects.

(e) Blastocyst - Blastula of Eutherian & Metatherian mammals is called blastocyst, because blastula is in the form of a cyst.

Blastula - Blastula of mammals is called blastocyst. In blastocyst all the embryonal cells occur in the form of solid mass called embryonal knob. Embryonal knob (inner cell mass) is covered by protective layer called trophoblast and it's cell just above the embryonal knob are called cells of Rauber (amniogenic cells). There occurs a cavity in between embryonal knob and trophoblast called albumin cavity (Blastocoel). It is filled with nutritive fluid absorbed from the wall of uterus. So albumin cavity is also nutritive- cavity.

3. Gastrulation

Gastrula: In gastrula stage rate of cleavage division is slow and ultimately stops at the end of gastrula. Gastrula stage is the most important stage in embryonic development because two main events take place during gastrula stage.
(a) Differentiation of blastomere: As a result of differentiation of blastomere three germinal layers i.e. ectoderm, mesoderm and endoderm are formed. Formation of three germinal layers is the significance of gastrula stage. All the preparation of differentiation of blastomere are completed in late blastula stage.

(b) Morphogenetic Movements: During gastrula stage blastomere perform amoeboid movement and reach to their definite place in embryo because after the gastrulation organogenesis has to start in embryo. Morphogenetic movement requires enormous energy. So respiratory activity of egg increases. Embryo consumes maximum O2 during gastrula stage.

Method of Gastrulation :
(a) Epiboly: Movement of ectoderm forming blastomere
(b) Emboly: Movement of mesoderm and endoderm forming blastomere.

(a) Epiboly: In epiboly, ectoderm forming blastomere undergo division to form new blastomere. New blastomeres perform amoeboid movement and cover the embryo from outside. Epiboly is clearly visible in animal with amphiblastula. In amphiblastula ectoderm is derived from outermost layer of dividing micromeres.

Fig: Process of Epiboly

These micromeres perform amoeboid movement and cover the megamere from outside. Megameres without any movement establish inside the embryo because they are heavily filled with yolk. At one definite place in embryo micromeres do not perform any movement to cover the megameres this place occurs in the form of depression is called gastrulation slit or future blastopore. It is formed in grey crescent area.

In some animals there occur special type of epiboly is called Apiauxis. In this process ectoderm forming micromere are received from two sources.

(1) By the division of micromere

(2) By budding of megamere. (Eg-Ctenophora. Annelida, Mollusca and Fishes)

(b) Emboly: Mesoderm and endoderm forming blastomere perform movement and establish inside the embryo. There are three methods of emboly.

(i) Invagination: Emboly mainly takes place by invagination in animal with simple coeloblastula. In coeloblastula a part of blastoderm invaginate in embryo, as a result, blastocoel cavity degenerates and new cavity forms outside to inside called archenteron. The opening of the archenteron is called the blastopore. Archenteron forms an alimentary canal. Blastopore forms mouth in protostome animals and anus in deuterostome animals. As a result of invagination blastomere which fills the blastocoel form mesoderm and endoderm. Blastomere of outer layer form ectoderm i.e. all the three germinal layer are formed by emboly in coeloblastula. Epiboly is absent in coeloblastula.

(ii) Involution : Rolling movement of blastomere. Emboly mainly takes place by involution in animal with amphiblastula.

  • In amphiblastula micromeres move from different directions towards future blastopore,this movement is called convergence.
  • After convergence, second step is involution (i.e. blastomere perform rolling movement and enter in blastocoel with the help of future blastopore) now all the blastomeres move in different direction in blastocoel, this is called divergence.
  • As a result of divergence blastocoel cavity degenerates and new cavity called archenteron is formed. Opening of archenteron is called blastopore.
  • All the involuted blastomeres are collectively called 'Chordamesoderm'. At the end of gastrula, outgrowth occurs in some megamere i.e. megameres evaginate outside and close the blastopore temporarily (yolk-pluge stage) yolkpluge stage indicates completion of gastrulation.

Fig: The process of Involution

(3) Poly-invagination or ingression: Emboly mainly takes place by poly-invagination in discoblastula, periblastula.

Fig: The process of poly-invagination Gastrulation in discoblastula:
In discoblastula, gastrulation takes place by two methods:

(i) Delamination : In delamination all the blastomere of blastoderm undergoes division. As a result new blastomere form and new blastomere fall on the floor of sub-germinal cavity. So embryo become double layered. Upper layer of blastomere is called epiblast and lower-layer of blastomere is called hypoblast. Hypoblast differentiates in endoderm.

(ii) Poly-invaginations: All the blastomere of epiblast undergo division to form new blastomere and new blastomere fall in subgerminal cavity form different direction and fill the subgerminal cavity. Blastomere which fill the subgerminal cavity are collectively called chorda-mesoderm. (chorda mesoderm forms mesoderm) Blastomeres which are left outside i.e. blastomere of epiblast form ectoderm. In such blastulas no new cavity is formed during gastrula stage i.e. archenteron formation does not occur during gastrula stage.

SPECIAL POINTS
1. The growth phase is the longest phase during male gametogenesis. But in human oogenesis, maturation phase is longest.

2. The acrosome of sperm are produced by golgibodies.

3. The smallest sperm is of crocodile and its size is 0.02 mm & largest sperm is of Discoglossus (2mm)

4. 74 days are required to complete the cycle of spermatogenesis in human being.

5. In 1 ml of semen, 20 to 120 millions of sperms are present in human being.

6. Deficiency in the number of sperms result in sterility which is known as oligospermia.

7. Absence of sperms in semen is known as azoospermia.

8. Formation of yolk in oogenesis takes place in the growth phase.

9. Largest egg is of Ostrich ( 16 cm long with its shell).

10. Although normal number of sperm are present in semen but if these are completely non motile. This condition is known as necrospermia.

11. Smallest egg in birds is of humming bird.

12. Due to the high mortality rate in lower animals, the production of egg is more.

13. The sequence of egg production is as follows.

Mammals < Aves < Reptiles < Amphibian < Pisces.

14 Cat and rabbit both are induced ovulator.

15. The life span of eggs in female reproductive organs in human being is 48 hrs.

16. The nucleus of egg is known as germinal vesicle.

17. At the age of 45-50 yrs. in female the ovulation process will stop which is known as menopause.

18. The spermiation (release of sperms from sertoli cells) in all sertoli cells occurs simultaneously.

19. Cortical granules are absent in rat.

20. Mosaic type of cleavage is found in the parasite Echinococcus granulosus.

Special features of some animals :
(a) Sperms of some animals are not having flagella:
Eg. (1) Ascaris - sperm is amoeboid

(2) Cray fish - star shaped, tail less sperm

(3) In crab and lobster the sperm are tail less and have three sharp processes.

(b) Biflagellated sperm :
Eg. In toad fish (Opsansus) head of many sperms unite together and form sperm boats. In Gastropods, the sperms are hexa-flagellated.

Smallest sperm - Crocodile (0.02 mm)

Largest sperm - Discoglossus (2 mm) in chordates and Drosophila in entire animal kingdom.

Shape of head part of sperms :
(i) Spherical - eg:- Teleostei

(ii) Lance shaped - eg:- Amphibia and Reptiles

(iv) Spoon shaped - eg.:- Mammals (in man)

Germinal layers and their derivatives
The following description gives an account of the respective organs formed by the three germ layers. Most of the organs are the product of combination of more than one germ layers.

Organs derived from ectoderm
1. Skin(epidermis) and their pigment cells.

2. Mucosal membrane of lips, cheek, gums, basal portion of mouth, some part of palate, nasal apertures.

3. Lower part of anal canal.

5. Labia majora and outer part of labia minora.

6. Anterior epithelium of cornea, epithelium of conjunctiva, ciliary body and iris of eyes.

7. Outer face of tympanic membrane, epithelium of labyrinth.

9. Hairs, nails, enamel of teeth

Derivatives of mesoderm
1. Connective tissues, superficial and deep fascia, ligaments, tendons, dermis of skin. (from dermatome)

2. Specialized connective tissues like adipose tissue, reticular tissues, bones, cartilages.

5. Heart, all blood vessels and blood cells.

6. Kidneys, ureters, urinary bladder, posterior urethra of female, upper glandular part of prostate.

8. Testes, epididymis, vas deferens and seminal vesicle, ejaculatory duct.

9. Pleural cavities, peritoneal cavity and pericardial cavity.

11. Cornea, sclera, choroid ciliary body and iris related material.

12. Microglia, duramater etc.

Derivatives of endoderm
1. Epithelial part of mouth, some part of palate, tongue, tonsils, pharynx, oesophagus, stomach, small and large intestine, upper part of anal canal.

2. Pharyngo-tympanic tube, middle ear, inner face of tympanic membrane.

4. Gall bladder, pancreatic duct.

5. Major portion of urinary bladder, complete urethra of female except posterior part, complete urethra of male except anterior and posterior part.

6. Whole inner part of vagina including inner face of labia minora.

(ii) Endocrine:-

(D) Islets of Langerhans In addition to the above, the glands of gastrointestinal tract, major part of prostate etc. are also formed by endoderm.


Chapter Notes - Human Reproduction, Class 12, Biology Class 12 Notes | EduRev

Human Reproduction

INTRODUCTION :

Embryology is the branch of biology which involves the study of all those processes, which take place during the development of an adult from the egg.

FORMATION OF GAMETES :

Follicle stimulating hormone stimulates gametogenesis. Besides this hormone vitamin E is also essential for gametogenesis. Deficiency of vitamin E leads to sterility. Vitamin A is also required for the formation of healthy gametes.

Gemetogenesis is divided in three stages :

As there are two types of gametes, the spermatozoa and ova, gametogenesis can be studied under two broad headings : spermatogenesis and oogenesis. Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova. Both spermatozoa and ova originate from primordial germ cells or PGCs, which are extra-gonadal in origin. In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm. Eventually, they migrate to the yolk sac endoderm, and ultimately, to the gonads of the developing embryo, where they undergo further development. You can recall that spermatogenesis occurs in the seminiferous tubules of the testes of oogenesis occurs in the follicles of ovary. Formation of gametes starts at puberty.

SPERMATOGENESIS

Spermoatogenesis : i.e. formation of sperms. In most of the animals spermatogenesis takes place in testes, (exception-earthworm). Mammalian testes contain seminiferous tubules and wall of seminiferous tubule is composed of germinal epithelium. It contains some special types of cells called primordial germ cells and these cells start spermatogenesis. On the basis of origin, primordial germ cells are extra embryonic mesodermal. Besides these cells, germinal epithelium contains some large sized cell called sertoli cells. Occurrence of sertoli cells is the unique feature of mammalian testis. Sertoli cells provide nutrition of developing sperm i.e. developing sperms are embedded in cytoplasm of sertoli cells and absorb nutrition. After maturation sperms comes out from sertoli cells and librate in seminiferous tubules.

Liberation of sperms from Sertoli cells is called spermiation.

Liberation of sperms from testes is called semination.

Liberation of sperms from body of male is called ejaculation.

Mammalian sperms are transferred to vagina of female by the process called insemination.Sertoli cells form 'blood testes barrier' and protect the sperm from immune system of the body. because

antibody may attach on haploid cells and destroy them. (Sperms are haploid and other cells of body are diploid).

Sertoli cells function as an endocrine gland i.e. secrete three type of hormones :

(i) Antimullerian hormone : function of this hormone is degradation of female gonads in male embryo. (In male seminal vesicle is the ruminant part of oviduct of female).

(ii) Inhibin hormone : Function of this hormone is to control excess secretion of pituitary gland to prevent the over-production of sperms.

(iii) Androgen binding protein : Function of this hormone is to concentrate testosterone in seminiferous tubules because testosterone is must for spermatogenesis in seminiferous tubules.

STEPS OF SPERMATOGENESIS

Spermatozoa are formed in the wall of the seminiferous tubules of the testes. The various cell-stages in spermatogenesis are as follows (the number of chromosomes at each stage is given in brackets)

An adult male produces over 10 12 to 10 13 sperm cells each day. These gradually move into the epididymis and the first portion of the vas deferens, where they undergo further maturation and are stored.

The spermatogonia (type A) or germ cells (44 X + Y) divide mitotically, to give rise to more spermatogonia of type A (spermatogenic lineage) and also spermatogonia of type B.

The spermatogonia (type B) (44 + X + Y) enlarge, to from primary spermatocytes (spermato cytogeneis)

The primary spermatocytes (44 + X + Y) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division. it reduces the number of chromosomes to half.

Each secondary spermatocyte has 22 + X or 22 + Y chromosomes. It divides to form two spermatids. This is the second meiotic division and this time there is no reduction in chromosome number.

Each spermatid (22 + X or 22 + Y) gradually changes its shape to become a spermatozoon. This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis/spermateleosis

In spermiogenesis first, of all nucleus of spermatid shift at one side. Except chromatin material, all
the structures come out from nucleus as a result nucleus become small and light in weight. Then several golgi vesicles gathered just above the nucleus. Some of the Golgi vesicles develop granules called
proacrosomal granules, such granule containing vesicle is called proacroblast. Rest of the golgi
vesicles are called golgirest, the golgirest dissolve in cytoplasm. All the proacroblast fuse and form large vesicle called acroblast and it's granule is called acrosomal granule. Acroblast arranged just above the nucleus. Then cytoplasm of spermatid starts moving towards posterior side. As a result, plasma membrane shrinks and gets attached to acroblast and nucleus. Due to this head of sperm is formed. Now centriole starts forming axonema, then all the mitochondria of spermatid are arranged in spiral order around axonema. In this way middle piece of sperm is formed. Now axonema elongates towards posterior side and forms tail. Cytoplasm of spermatid continuously flows towards posterior side.

Structure of sperm :

The spermatozoon has a head, a middle piece and a tail. The head is covered by cap the acrosomic cap, anterior nuclear cap, or galea capitis. Acrosome is a bag like structure filled with lytic enzymes called spermlysins. In the anterior part of middle piece neck is present. The neck is narrow : it contains a
proximal & distal centriole (or Basal body). An axial filament begins just behind this centriole, it passes through the middle piece and extends into the tail. At the point where the middle piece joins the tail, this axial filament passes through a ring-like structure called the annulus (or ring centriole or zensons ring). That part of the axial filament which lies in the middle piece, is surrounding by a spiral sheath made up of mitochondria. (Nebenkern sheath)

Nuclear part of head of spermatozoa consist of chromatin (mostly DNA) that is extremely condensed. It contains a basic nature protein called protamin.

The basal body is made up of nine segmented rod like structures each of which is continuous distally with one coarse fibril of the axial filament.

The axial filament, that passes through the middle piece and most of the tail, is actually composed of several fibrils arranged. There is a pair of central fibrils, surrounded by nine pairs (doublets) arranged in a circle around the central pair (9+ 2). This arrangement of one central pair of fibrils surrounded by nine doublets is covered by nine solid protein (making the arrangement as 9 + 9+ 2).

In the proximal tail part it is covered by only two solid protein fibres (therefore arrangement is 2 + 9 + 2), while end part of tail has no protein covering (therefore arrangement is 9 +2). Immediately outside the fibrils there is a fibrous health.

Middle piece (also called as the energy chamber) is surrounded by spirally arranged mitochondria (Nebenkern sheath). Finally, the entire sperm is enclosed in a plasma membrane.

Like spermatogenesis oogenesis process also can be divided into three stages :

(A) Multiplication

(B) Growth phase

(C) Maturation phase

(A) Multiplication phase : In this stage primordial germ cells or ovum mother cells repeatedly divide by
mitosis to form large number of diploid oogonia.

This process completes in embryo stage of female in most higher animals.

(B) Growth phase : Like spermatogenesis, in this process oogonia grow in size and form primary oocytes. The growth phase is the longest phase oogenesis (except humans). During growth phase size of egg increases many times.

During growth phase several changes occur in egg and all these changes are classified in 2 sub-stages

Previtellogenesis

Vitellogenesis

Previtellogenesis :

During previtellogenesis, changes occur in nucleus and cytoplasm of egg.

Amount of nucleoplasm increases in nucleus.

Number of nucleolus increase in nucleus.

Formation of lamp brush chromosome starts.

Activity of DNA increases in nucleus, as a result DNA become highly active and rapidly synthesizes different types of RNA. Increased activity of DNA is called as Gene redundancy/Gene amplification. Due to all these changes, size of nucleus increases and nucleus becomes vesicular. This vasicular nucleus is called germinal vesicle .

In cytoplasm, rate of protein synthesis increases. Cytoplasm rapidly synthesises different type of protein and enzyme. Due to more availability of protein and enzymes, synthesis of new protoplasm takes place and size of egg increases.

Number of cell organelles increase in cytoplasm, specially endoplasmic reticulum, golgi-body and mitochondria Mitochondria become very large in number so mitochondrial clouds are found in cytoplasm of egg.

Later on all these 3 cell organelle (golgi body, endoplasmic reticulum, mitochondria) are arranged in the form of ring around the nucleus, it is called as Balbiani vitelline ring. In the stage golgi body of egg secretes a membrane around the egg which is it called as vitelline membrane. A space appears in between plasma membrane of egg and vitelline membrane called as perivitelline space, It is filled with a fluid called perivitelline fluid.

At the end of previtellongenesis endoplasmic reticulum disappear. Golgi bodies gets converted into corticle granule. Corticle granules are filled with mucopolysacharide. Large number of change occur in mitochondria also.

Vitellogenesis : During vitellogenesis egg stores food in the form of yolk.

Some part of yolk is synthesized in egg only, but major part of yolk is received from liver. Yolk received from liver is less viscous and is therefore soluble but this type of food cannot be stored for long periods (because it easily gets converted into simple form)

So mitochondria of egg with the help of kinase enzyme make the yolk more viscous and insoluble.
2 types of yolk is found :

Granular yolk occurs in the form of fine granules.

Yolk platelets occur in the form of plate disc like granule.

eg. Deuterostomia animals (higher animals)

Chemical composition of yolk :

1.Most abundant compound in yolk is phospholipid

Most common phospholipids is lecithin.

2. Yolk contains different type of protein :

Simple protein : Albumin, Globin, Globulin

Phosphoprotein : Phosvitene, Ovovitelline

3. In yolk, least amount of substance found is carbohydrate.

(C) Maturation phase : Oogenesis (Fig. 2) takes place in the ovaries. In contrast to males the initial steps in egg production occur prior to birth. By the time the foetus is 25 weeks old, all the oogonia that she will ever produce, are already formed by mitosis. Hundreds of these diploid cells develop into primary oocytes, begin the first steps of the first meiotic division, proceed up to diakinesis, and them stop any further development. The oocytes grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives very little amount of cytoplasm but one full set of chromosomes.

In humans (and most vertebrates), the first polar body does not undergo meiosis II, whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. However, it then stops advancing any further, it awaits the arrival of the spermatozoa for completion of second meiotic division. Entry of the sperm restarts the cell cycle breaking down MPF (M-phase promoting factor and turning on the APC (Anaphase promoting complex). Completion of meiosis II converts the secondary oocyte into a fertilized egg or zygote (and also a second polar body)

Ova are derived from oogonia present in the cortex of ovary. Some important differences between oogenesis and spermatogenesis are

(i) Whereas one primary spermatocyte gives rise to four spermatozoa, one primary oocyte forms only one
ovum.

(ii) When the primary spermatocyte divides, its cytoplasm is equally distributed between the two secondary
spermatocytes formed. However, when the primary occyte divides, almost all its cytoplasm goes to the
daughter cell which forms the secondary oocyte. The other daughter cell (first polar body), receives half the chromosomes of the primary oocyte, but almost no cytoplasm.

The first polar body is, therefore, formed merely to get rid of unwanted chromosomes.

TYPES OF EGGS

(A) On the basis of amount of yolk

(i) Alecithal or Microlecithal or Oligolecithal eggs &ndash The amount of yolk is very small or absent in these types of eggs. (oligolecithal, or microlecithal or alecithal).

The term 'alecithal' (absence of yolk eg. Man) was given by Kent. And term microlecithal' (eg. Urchin) was given by Tori.

Examples :- Egg of Amphioxus, Eutheria, Metatheria and sea &ndash urchin.

(ii) Mesolecthal Eggs : - In this type of egg, the amount of yolk is moderate i.e medium ,neither more nor less.

Example : - Eggs of Amphibia, Petromyzon and lung-fishes.

(iii) Polylecithal or Macrolecithal or Megalecithal eggs : -

Eggs are with large amount of yolk. e.g. Bird's egg , Insect's egg.

(B) On the basis of distribution of yolk :

(i) Isolecithal or homolecithal eggs : The yolk is evenly or homogenously distributed in these eggs. eg. : micro, oligo or alecithal eggs.

(ii) Telolecithal eggs : The yolk is concentrated in one part of the egg.

eg. Mesolecithal eggs of amphibia. (Moderately telolecithal)

Discoidal eggs : A type of telolecithal and megalecithal eggs. Where the yolk is in enormous quantity and concentrated in one part of the egg. Thus only a disc of cytoplasm called germinal disc remains in the egg which is located at the other pole of egg. (Heavily telolecithal)

eg. : Eggs of reptiles, birds and prototherian mammals

(iii) Centrolecithal eggs : Megalecithal eggs where the enormous amount of yolk is located in the centre an cytoplasm is in the form of superficial layer around the yolk.

(C) Classification of Eggs on the basis of Shell : -

On the basis of shell, eggs are of 2 types : -

(i) Cleidoic eggs : - Eggs surrounded by a hard shell are known as cleidoic eggs. These eggs are found in those animal which have a terrestrial mode of life or which lay eggs on land.

These eggs have more amount of yolk. These are adaptations to terrestrial mode of life. Shell prevents the egg from dessication.

(ii) Non-Cleidoic eggs : -

Eggs which are not surrounded by a hard shell are called non-cleidoic eggs.

eg. : - all viviparous animals (Mammals) and all oviparous animals which lay eggs in water (Amphibians). Reptilia eggs are called leathery eggs.

STRUCTURE OF OOCYTE

The nucleus of egg is also called germinal vesicle.

Oocyte is surrounded by membranes termed as the egg-membranes.

Oocyte/Ovum along with the egg-membrane are termed as the egg.

Egg = Ovum/Oocyte + Egg membrane.

Majority eggs are oval but the eggs of insects are long and cylindrical. Smallest eggs are of 50m in polychaeta and the largest eggs are of an Ostrich.

Classification of egg - membranes :

On the basis of origin, egg-membranes are of 3 types : -

(1) Primary egg membrane :

This membrane is secreted by the oocyte itself.

eg. Vitelline membrane, Zona Pellucida (mammals)

(2) Secondary egg membrane :

This is found outside the primary egg membrane and is secreted by the ovary.

(3) Tertiary egg membrane :

The is present outside the primary egg membrane. It is either secreted by the oviduct.

Functions of Egg-membranes

(iii) To provide buoyancy to the amphibian eggs

Different types of eggs

(I) INSECT EGG

Eggs of insects are megalecithal or polylecithal in them yolk is present in the centre, so the eggs are also centrolecithal.

Two egg membranes are present here, inner vitelline membrane (primary) and outer chorian (secondary).

The sperm enters the egg through micropyle because on the head of insect sperm acrosome is absent.

The Cytoplasm here is found in two parts :

(a) Central cytoplasm : - It is present in a very small amount in the centre of the egg. Egg nucleus is located in it.

(b) Peripheral Cytoplasm : - It is present in a very small amount along the periphery of the egg.

(II) FROG'S EGG

Eggs of frog are moderately Telolecithal & Mesolecithal

Two types of egg membranes are found in frogs egg :

(i) Vitelline membrane : This is primary egg membrane which is secreted by the ovum around itself.

(ii) Jelly coat : This is tertiary egg membrane, secreted by oviduct. Jelly coat has air bubbles trapped in it due to which it floats on water. This group of frogs egg is called spawn, Jelly coat is bitter in taste so enemies do not eat it.

Secondary egg membrane is absent in frog's egg.

Internal part of the egg is divided in two parts :

(i) Animal pole : This part has cytoplasm, egg nucleus in also located in this part. In the cytoplasm melanin granules are found which prevent the egg from harmfull radiations. They also help in protection of egg by camouflage.

A sperm always enters into the ovum at some point in animal hemisphere. This point is normally other than the animal pole itself.

As the sperm enters into the ovum, taking some pigment granules with it, a grey, crescent shaped region appears in the equatorial zone geometrically opposite the sperm entrance point. This region is called grey crescent. It is formed due to movement of some pigment granules away from it towards sperm entrance point.

(It marks the dorsal side of future embryo). This area of sperm entrance point marks the anterior side of future embryo. The side diagonally opposite to it in the vegetal hemisphere marks the future posterior side. Thus the sperm entrance establishes the anteroposterior and dorsoventral axis as well as bilateral symmetry of future embryo.

(ii) Vegetal pole : yolk is concentrated in this part of egg.

(III) CHICK EGG

These eggs are megalecithal or polylecithal and discoidal eggs.

In these eggs, yolk is present in the centre of the egg in the form of a dense mass. The cytoplasm of the egg is in the form of a disc above the yolk, which is termed as the germinal-disc.

Yolk is of 2 types i.e. yellow yolk and white yolk.

Yellow &ndash yolk has more amount of phospholipids and is secreted during the night. White yolk has less amount of phospholipids and is secreted during the day time. Central white part of yolk is called latebra.

Both the types of yolk are arranged in alternate and concentric layers.Pander was the scientist, who discovered the 3 germinal layers i.e. Ectoderm Mesoderm and Endoderm in chick - egg.

Around the egg, porous shell of CaCO3 is present which is secreted by the cells of the oviduct.

In between the vitelline membrane and the shell membrane albumin is filled which is also called the white of egg. It contains 13% proteins.

Thick albumin-fibres termed as ''Chalaza'' are present in the albumin part of egg.

(IV) EGG OF MAMMALS

Mammalian eggs have very less amount of yolk, so the eggs are oligolecithal and isolecithal or microlecithal and homolecithal.

The egg has 2 egg-membranes : -

(1) Zona Pellucida : - This is a transparent membrane like covering and is a primary membrane secreted by the ovum/oocyte itself.

(2) Corona radiata : - This is a layer of follicular cells'' and these cells are attached to the surface of egg through ''hyaluronic acid'' This is a secondary membrane, which is secreted by the ovary. These eggs don't have tertiary membrane.

Mammalian eggs are approx 0.1 mm in size.

FERTILIZATION

The process in which union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum takes place thus diploid zygote is formed, is called fertilization.

Fertilization has following processes : - The union of male and female gametes is called Syngamy. Where as intermixing of their cytoplasm is called plasmogamy. The fusion of pronuclei of sperm and ovum is called karyogamy. The intermingling of their chromosomes is called amphimixis.

Due to fertilization , a diploid zygote is formed, by the union of two different types of gametes.

SITE OF FERTILIZATION :

(A) INTERNAL FERTILIZATION - Fertilization in the body (i.e., genital organs of animal) is called internal fertilization. In this type of fertilization, sperms are discharged by male directly into the genital tract of female after coitus.

Whole process of fertilization takes place within the body of female. This is the most common
adaptation in terrestrial animals.

Examples : - Aschelminthes, reptiles, birds and mammals

(B) EXTERNAL FERTILIZATION - External fertilization takes place outside the body of females i.e., in water. Example : - In most of the invertebrates, some protochordates, amphibian and most of the fishes.

TYPES OF FERTILIZATION :

(a) Self fertilization - This process takes place in the body of single animal i.e., fusion of male and female gametes produced by male and female organs of the same animal. This is called self-fertilization. This is possible only in bisexual or hermaphrodite animals.

Examples : - Animals of phylum porifera and most of the species Hydra.

(b) Cross-Fertilization &ndash Fertilization takes place between two (male & female) different animals of same
species.

This is called cross fertilization.

This process is found in all unisexual animals. These animals are also called dioecious animalsCross-fertilization is also found in most of the bisexual or hermaphrodite animals because in these animals male genital organs develop first. This condition is called protandrous condition. In some of the species female organs develop first, this condition is called protogynous condition e.g. sponges.

MECHANISM - We can understand the process of fertilization in following steps.

1. Movement of sperms towards ovum.

2. Entrance of sperms in the ovum.

Approach of sperm towards the egg &ndash it is a chance factor, so sperms perform random (directionalless) movement. To increase the chances of approach of sperm towards egg there are mainly two adaption

(a) Number of sperms is very high : - e.g. In man 20 to 120 million sperms are present per cubic mm of semen.

(b) Egg is 1000 times larger than sperm

Some special proteins are found on the surface of egg and sperm to help in fertilization.

According of Lillie, chemicals named as ''fertilizins'' are found on the surface of egg. Fertilins are glycoproteins or acid mucopolysaccharides. According to Ballinsky, an acidic protein named as Antifertilizin is present on the surface of sperms. ''Fertilin'' proteins are also present on sperm surface.Both the proteins are specific for a particular species. Antifertilizin present on sperm of a particular species will react with fertilizin of present on egg of the same species of animals.

If we place some eggs of sea-urchin in sea-water, this sea water becomes viscous, this is called egg-water. When some sperms come in contact with this egg water, sperms adhere with each other. It is called agglutination.

Here the reaction of fertilizin (dissolved in water from egg) and antifertilizin of sperm is observed clearly.

Fertilizins behave like lock and antifertilizins behave like keys.

Fertilization is always intraspecific.

According to Washerman and Sailing (1989) a specific pair of protein molecules is found on the surface of mammalian sperm, which can recognize specific carbohydrates and proteins in ZP3 region of zona pellucida. The bindin protein of sperm reacts with these molecules to initiate the changes in acrosome.

A specific sugar galactose remains attached with ZP3 glycoprotein. The sperm fails to recognize the ovum of its own species, if this sugar is removed from zona pellucida.

In addition to these glycoproteins, there are some hormones also, which help in fertilization.

The hormones present at the surface of sperm are called androgamones. These are of two types. Androgamone first & androgamone second.

Androgamones I help in the energy conservation of sperms.

Androgamones II dissolve the gelatinous coverting present all over the egg.

Hormones present at the surface of egg are called gyanogamones these are of 2 types.

(a) Gyanogamones I - this hormone neutralizes Androgamone I and activates sperm to move

(b) Gyanogamones II - It makes sperm head sticky.

Enzyme of acrosome (Hyaluronidase and sperm lysins) dissolve the egg membrane. This is called acrosomal reaction. As a result sperm head make the contact with the plasma membrane of egg, now inner membrane of acrosome evaginates outside and form rigid tube is called acrosomal filament. Acrosomal filament provide stimulus to plasma membrane of egg and due to stimulus of sperm, egg is induce for fertilization. Mammalian sperms do not form this type of filament because mammalian sperms are highly active and provide stimulus to plasma-membrane of egg without any filament. Mammalian sperms acquire activity at two places. First-epididymis and second-vagina. Vaginal secretion make the sperm highly active and sperm acquire capacity of fertilization is called capacitation.

Activation of egg : Due to stimulus of sperm an enzyme is induced in plasma-membrane of egg it
is called adenyl cyclase enzyme and function of this enzyme is to catalyze C-AMP in egg cytoplasm.
C-AMP is the second messenger. Cyclic AMP receive stimulus from plasma membrane of egg and transfers it in egg cytoplasm and induces all the response of egg for sperm.

All the response of egg for sperm are collectively called gyanogenesis.

Due to stimulus of sperm, permeability of plasma membrane of egg increases specially for k + and Ca 2+ ions. Function of Ca 2+ ions is to inactivate the cytostatic factors in egg. As a result egg is now ready for cleavage (In egg cytoplasm special type of protein called cytostatic factor are present these factor prevent the cleavage in unfertilized egg)

Due to stimulus of sperm, H + &ndash Na + pump activates and induces the plasma-membrane of egg. Function of this pump is to continuously influx H + ions and outflux Na + ions. As a result concentration of H + ion increases in egg cytoplasm and develops an acidic medium. In acidic medium, proteolytic enzyme become active and liberate the m-RNA from informosome. These m-RNA become active and rapidly synthesize different types of protein and enzymes. Due to more availability of protein and enzymes metabolic activity of egg increases.

Response of egg :

(1) Due to stimulus of sperm, meiosis-II is induced in human egg by excluding second polar body becoming mature ovum.

(2) At the point of contact with sperm and plasma-membrane of egg a cone-like structure is formed called reception cone. After some time reception cone sinks in egg cytoplasm along with sperm (entry of sperm is a type of phagocytosis). With the entry of sperm all the cortical granules burst and secrete a membrane around the egg is called fertilization membrane (cortical reaction). It is secreted on inner surface of primary egg membrane and perivitelline space become more wide and amount of perivitelline fluid is also increase. Function of perivitelline fluid and fertilization membrane is to prevent the entry of sperm in egg. so normally only one sperm enter inside the egg (monospermy). Sometimes more than one sperm enter inside the egg (polypermy).

Two types of polyspermy are found in nature.

(1) Pathological polyspermy : In it the nuclei of all the sperms fuse with egg nucleus. In such type of condition embryo development does not occur. (Due to polyploidy condition)

(2) Physiological polyspermy : In physiological polysermy nucleus of only one sperm fuses with egg nucleus and rest of the sperm die in egg cytoplasm. Dead sperm are called merocytes. In physiological polyspermy normal embryo development occurs.

Polyspermy is absent in human beings. Polyspermy mostly occurs in megalecithal eggs.

Fate of sperm in egg - In majority of animals, only head and middle piece enter inside the egg and tail is left outside.

In mammals, whole sperm enters in the egg.

In some animals, only head of sperm enters in the egg tail and middle piece remain outside
e.g. Hydra, Neries etc. After entering inside the egg, sperm rotates by 180°. All the structure of sperm dissolve in egg cytoplasm except sperm nucleus and proximal centriole.

The centriole of egg itself degenerates at the time of second maturation division. So proximal centriole of sperm starts division, it divides into 2 daughter centrioles, which migrate towards opposite pole and start forming spindles.

Fat of sperm nucleus : -

The nucleus of sperm absorbs water from egg cytoplasm and becomes enlarged. Now it is called male pronucleus.

After meiosis &ndash II egg nucleus occur in the form of scattered vesicles then it is called as karyomeres and after some time ll the karyomeres assembled to form complete nucleus is called female pronucleus.

Male pronucleus and female pronucleus migrate through definite routes and come close to each other. These routes are called fertilization path. (It has following parts)

(1) Sperm penetration path - Male pronucleus for some distance, moves at the equator of egg. This is called sperm penetration path.

(2) Sperm copulation path - Male pronucleus starts migrating towards female pronucleus.

(3) Egg copulation path - Female pronucleus migrates towards male pronucleus. Both the pronuclei come close to each other.

(4) Cleavage path - Both the pronuclei move together to their final position which is somewhere in animal pole. At this final position nuclear membrane of both the pronuclei degenerate and chromosomes of male and female pronuclei form pairs. The mixture of male and female chromosomes is called amphimixis.

Amphimixis was discovered by O.Hertwig in the eggs of sea &ndash urchin.

Newport was of all first observed the entry of sperm into the egg.

Significance of Fertilization

1. Oocyte completes its second maturation division on coming in contact with the sperm.

2. Amphimixis process leads to the formation of diploid zygote to restore the normal diploid number of the chromosomes.

3. The centriole of sperm after entering into egg induces the egg to undergo cleavage.

4. The paternal and maternal characters are transmitted to the off springs through the process of fertilization.

5. The peripheral changes occurring in the egg prevent the further entry of sperm into the ovum, thus checking polyspermy.

PARTHENOGENESIS OR VIRGINAL DEVELOPMENT

The development of embryo without fertilization is called parthenogenesis. The animals which are formed by unfertilized eggs are called parthenotes.

The discovery of parthenogenesis was done by Charles Bonet in the eggs of sea-urchins.

Parthenogenesis is of two types &ndash

(a) Natural parthenogenesis -

Some animals show parthenogenesis by nature e.g. Honey bees, wasps, ants, grass-hoppers, ticks, mites and sea-urchins. Natural parthenogenesis is of 2 types : -

(i) Haploid parthenogenesis or Arrhenotoky : - In this case eggs are formed by meiosis. Eggs are haploid, they have the power of fertilization sometimes male animals are developed by unfertilized eggs. In Honeybees, unfertilized eggs develop into males (drones), and fertilized eggs develop into queen and soldiers. Thus male honey bees are always haploid and queen with soldiers are always diploid.

(ii) Diploid parthenogenesis or Thelytoky : - In this case, eggs are formed without meiosis division. Eggs are diploid they do not have the power of fertilization.

Diploid eggs give rise to female generation only. Male members are absent in these species.

Examples : - Lacerta sexicola armenica (lizard), Caresius aratus gibelio (Fish).

Diploid parthenogenesis may also be divided into two types &ndash

(A) Ameiotic Thelytoky - In this type of parthenogenesis, during oogenesis first meiotic division does not take place but second meiotic division occurs as usual. In this situation the ovum still remains diploid. These ova, when reproduce parthenogenetically give rise to diploid off springs. For example, Trichoniscus, Daphnia, Daphnia pulex etc.

(B) Meiotic thelytoky - If the eggs are formed by normal oogenesis process, but by one or other reasons the eggs retain their diploid chromosomal number, then the parthenogenesis is called meiotic thelytoky. It may happen because of autofertilization. Some species of order &ndash Lepidoptera exhibit this type of parthenogenesis.

In some animals parthenogenesis alternates with normal sexual reproductive cycle. This is called cyclic parthenogenesis e.g., Honey bee.

In some animals, development of animals is always by parthenogenesis and sexual reproduction is absent in these species. This is called complete parthenogenesis. Males are absent.

Example - Lacerta sexicola armenica (lizard)

(b) Artificial parthenogenesis : -

This type of parthenogenesis is done by artificial methods. Artificial parthenogenesis is done by putting eggs in different atmospheres or by giving special stimulus to the eggs. Different artificial methods used for this purpose are as follows -

(1) If we place eggs in brine or salt solution, KCl solution. Then eggs show parthenogenesis e.g. eggs of sea-urchins.

(2) By short exposure of radiations on eggs or exposure of silk insect egg to sunlight.

(3) If eggs are given shocks of temperature.

(4) If eggs are pierced by needle dipped in the blood of same animal. The eggs of frog show parthenogenesis by this method.

The term 'Cleavage' was given by ''Von Baer''.

In fertilized egg or activated egg, the egg undergoes repeated cell divisions which occur rapidly producing a multicellular structure without changing its size. All these mitotic cells divisions collectively called cleavage or segmentation. Due to the process of cleavage, a single celled zygote, through a successive mitotic cell divisions changes into a complex multicellular structure. Cells produced as a result of cleavage are termed as blastomeres. The total size of the embryo remains the same. Though the number of blastomeres as a result of mitotic cell divisions increases, the size of blastomeres gradually decreases are compared to parent cell. Interphase stage is very short in cleavage. In interphase only DNA duplication and histone protein synthesis takes place up to some extent. In the interphase of cleavage only 'S' phase is present, G1 & G2 phases are absent. Protein synthesis and RNA synthesis do not occur during this interphase. Nucleolus is absent in the nucleus of blastomeres. Size of blastomeres decreases during cleavage. When size of blastomere becomes equal to that of size of somatic ells, the divisions of cleavage are stopped. Only normal cell division take place. Cleavage can be observed till onset of gastrula stage. After gastrulation, cleavage is completely checked. Nucleous appears first in gastrula stage. The consumption of oxygen is increased during cleavage.

CLEAVAGE PLANE :

The traveling path of cleavage furrow in fertilized egg is called cleavage plane. Different animal eggs show different cleavage planes : -

PATTERNS OF CLEAVAGE :

(a) Radial Cleavage - In this pattern, cleavage furrows are straight and form right angle with each other. In
this case I, II cleavages are meridianal, which are at right angle to each other. III cleavage is equatorial In this way 8-celled octate is formed. In radial cleavage, 4 blastomeres of upper tier and 4 blastomers of lower tier are on sampe plane i.e, Blastomeres are arrangement in radial symmetry in the beginning.

(b) Biradial Cleavage - In this pattern, first two cleavages are meridianal and at right angle to each other all III cleavage is vertical. In 8-celled stage 4 blastomeres of central zone are bigger and 4 blastomeres of peripheral region are smaller.

Examples : - In the eggs of Ctenophora.

(c) Bilateral Cleavage - In this patterhn, first two cleavages are meridianal in same plane and III Cleavage is transverse. i.e., embryo shows bilateral symmetry in 8-celled stage. The blastomeres of one side are smaller and blastomeres of other side are larger.

Examples : - In the eggs tunicata, cephalochordata, amphibian and amphioxus.

(d) Spiral Cleavage - The cleavage furrow passes obliquely. In this pattern 4 blastomeres of lower tier rotate clockwise or anticlock wise. If this rotation is clockwise, then it is called dextral spiral cleavage e.g. In Mollusca. If this rotation of lower tier blastomeres is anti-clockwise, then pattern is called sinistral spiral cleavage, e.g. In helminthes and annelida.

CLASSIFICATION OF CLEAVAGE :

On the basis of fate of blastomeres : -

A. Determinate Cleavage - In this pattern of cleavage, the fate of blastomeres is fixed, determined i.e. each blastomere forms a particular portion of embryo. If (by certain reason) any blastomere is damaged or destroyed, then the part of embryo (which would have development from that blastomere) will be absent e.g., Nematoda, Annelida, Mollusca and Some chordates like amphibian & ascidians.

B. Indeterminate Cleavage - In this type of cleavage, the fate of blastomeres is not definite. All the blastomeres form all the parts of embryo. If some blastomeres are lost, no loss is observed in this embryo. If in the early stages of cleavage, the embryo is cut into small pieces, then each piece of embryo will develop into a complex embryo, and all the embryos are identical. So identical twins are monozygous. This is the basis of embryo cloning.

CLASSIFICATION OF CLEAVAGE :

On the basis of amount of Yolk : -

A scientist named Balfour gave a law. According to him, rate of cleavage is inversely proportional to amount of yolk present in the egg. The yolk present in egg, disturbs the rate of cleavage. The rate of cleavage is slow in that part of egg, in which amount of yolk is more, and the rate of cleavage is faster in the portion of egg in which yolk is in lesser amount. Mostly cleavage is of 2 types : -

A. Complete or holoblastic : - When cleavage furrow passes through the egg completely. As a result of this the whole egg divides. Holoblastic cleavage is found in all the eggs except megalecithal eggs. The whole egg divides into blastomeres. No part of egg remains undivided. It is of 2 types : -

(a) Equal holoblastic cleavage - In those eggs, in which amount of yolk is less and it is distributed evenly in the egg, cleavage occurs in whole egg, blastomeres (So formed) are of same size. All the parts of egg show same rate of cleavoage.Example : - It is found in microlecithal and isolecithal eggs.

(b) Unequal holoblastic cleavage - In those eggs in which amount of yolk is medium and it is distributed unevenly in the egg. The blastomeres are bigger and less in number where the concentration of yolk is higherin the egg. The part of egg which contains small amount of yolk, blastomeres here formed are smaller and more in number. The bigger blastomeres are called megamere and smaller blastomere are called micromeres.

Examples : - Unequal holoblastic cleavage is present in mesolecithal and telolecithal eggs and human eggs.

B. Meroblastic cleavage - This cleavage is found in megalecithal eggs, in which amount of yolk is large. Cleavage does not occur in that part of egg, where yolk is present cleavage occurs only Cytoplasmic part, yolk remains undivided. Meroblastic cleavage is of 2 types on the basis of distribution of yolk in egg.

(a) Discoidal meroblastic cleavage - Cleavage occurs only in blastodisc of egg. This is mainly found in megalecithal or polylecithal eggs, because in these eggs, cytoplasm is found in the form of a disc.

Examples : - Reptilian eggs and birds eggs.

(b) Superficial meroblastic cleavage &ndash In insect egg, central cytoplasm shows free central division, due to which so many nuclei are formed. All these nuclei migrates towards peripheral cytoplasm.
Cleavage occurs only in peripheral region. As a result of this, a superficial layer of blastomeres is formed around the yolk. This type of cleavage is also called superficial meroblastic cleavage.

Example - In centrolecithal eggs

Significance of Cleavage -

1. There is no change in shape and size developing embryo till blastula stage comes. Till then it remains
just like undivided egg in shape.

2. As a result of cleavage, unicellular zygote changes into multicelluar structure.

HUMAN EMBRYOLOGY

Fertilization-

Fertilization is the union of two opposite types of gametes, spermatozoa and ova. The semen is a mixture of spermatozoa and accessory fluids. Once deposited with in the vagina, the spermatozoa proceed on their journey into and through the uterus and on up into the oviducts. Although spermatozoa can swim several millimeters each second, their trip through the uterus and to the oviducts requires on increase in their motility.

On the first hand, ejaculation of semen in the vagina triggers motility of spermatozoa. This is aided further by muscular contraction of the walls of the uterus and the oviducts. An additional increase in sperm motility occurs due to activation of the sperm by the viscous liquid secreted from the secreted cells of the epithelial lining of oviduct mucosa. This phenomenon of sperm activation in mammals is known capacitation. It takes about 5-6 hours for capacitation.

Before fusion of a spermatozoan with the egg, the spermatozoa are to penetrate a few barriers, the egg membranes, which cover the egg. The activated spermatozoa undergo acrosomal reaction and release varies chemicals, like hyaluronidase that acts on the ground substances of follicle cells, corona penetrating enzyme that dissolves corona radiata, and zona lysine which perforates the zona pellucida. All these chemicals are contained in the acrosome, located at the tip of the sperm head, and are collectively termed sperm lysins. An average human ejaculate of 3-4 ml of semen contains 80-100 million spermatozoa. Out of these, only one will succeed in entering the egg and fertilizing it. Fertilisation of egg with only one spermatozoan is known as monospermy.

While the ovarian follicle is growing, the oogonium within it undergoes maturation. The oogonium enlarges to form a primary oocyte. The primary oocyte undergoes the first meiotic division to shed off the first polar body and becomes a secondary oocyte (fig. 5.2A). At the time of ovulation, the second meiotic division is in progress and a spindle has formed for separation of the second polar body (fig. 5.2B)

At this stage the 'ovum' enters the infundibulum of the uterine tube and passes into the ampulla (fig. 5)

Fertilization of the ovum occurs in the ampulla of the uterine tube. One spermatozoon pierces the zona pellucida and enters the ovum. In response to egg sperm binding two things happen

(1) depolarization of egg membrane

(2) cortical reaction i.e. formation of fertilization membrane to prevent polyspermy.

(After one spermatozoon has entered the ovum other spermatozoa cannot enter it). After the entry of the spermatozoon, the second polar body is extruded. The chromosomes of the ovum now assume the shape of a nucleus called the female pronucleus. At the same time the head of the spermatozoon (which it will be remembered is formed from the nucleus) separates from the middle piece and tail, and transforms it self into the male pronucleus (Fig. 5.2 D).

The male and female pronculei meet, but they do not fuse to form one nucleus. Their nuclear membranes disappear and their chromosomes become distinct. It will be recalled that each pronucleus has 23 chromosomes so that the fertilized ovum now has 46 chromosomes in all (fig 5.3 A).

Each of these 46 chromosomes splits into two (fig. 5.3B). Meanwhile, a spindle has formed, and one chromosome of each pair moves to each end of the spindle (as in mitosis). Leading to the formation of two daughter cells fig. (5.3C). This is called the two-cell stage of the embryo. Note that stricktly speaking there is no one-cell stage of the embryo (fig. 6A)

Important points note at this stage are that :

(i) the two daughter cells are still surrounded by the zona pellucida :

(ii) each daughter cell is much smaller than the ovum. As subsequent divisions occur the cells become smaller an smaller until they acquire the size of most cells of the body.

The two cells formed as described above undergo a series of divisions. One cell divides first so that we have a Ɖ-cell' stage of the embryo (fig.6B) followed by a Ɗ-cell' stage (fig. 6C), a 5-cell, stage etc. This process of subdivision of the ovum into smaller cells is called cleavage. These cells are called blastomeres.

As cleavage proceeds the ovum comes to have 16 cells. It now looks like a mulberry and it called the morula (fig. 6D). It is still surrounded by the zona pellucida. If we cut a section across the morula we see that it consists of an inner cell mass that is iscompletely surrounded by an outer layer of cells, (Compaction). The cells of the outer layer will later give rise to a structure called the trophoblast (fig. 7A). The inner cell mass gives rise to the embryo proper, where as the cells of the trophoblast help to provide nutrition to the embryo.

Some fluid now passes into morula from the uterine cavity, and partially separates the cells of the inner cell mass from those of the trophoblast (fig. 7B). As the quantity of fluid increases, the morula acquires the shape of a cyst. The cells of the trophoblast become flattened and the inner cell mass comes to be attached to the inner side of the trophoblast on one side only (fig. 7C). The morula has now become a blastocyst. That side of the blastocyst to which the inner cell mass is attached is called the embryonic or animal pole. While the opposite side is the vegetable or abembryonic pole.

Function of the zona pellucida -

The trophoblast has the property of being able to stick to the uterine (or other) epithelium and its cells have the capacity to eat up other cells. As the embryo travels down the uterine tube, and the upper most part of the uterine cavity it is prevented from 'Sticking' to the epithelium by the zona pellucida.

FORMATION OF GERM LAYERS (GASTRULATION) :

As the blastocyst develops further, it gives rise not only to the tissues and organs of the embryo, but also to a number of structures that support the embryo and help it to acquire nutrition. At a very early stage in development, the embryo proper acquires the form of a three-layered disc. This is called embryonic disc (also called embryonic area, embryonic shield, or germ disc) The three layes that constitute this embryonic disc are.

(ii) Extoderm (ecto = outside)

(iii) Mesoderm (meso = in the middle)

These are the three germ layers. All the tissues of the body are derived from one or more of these layers. We have seen that the blastocyst is a spherical cyst lined by flattened trophoblastic cells, and that inside it there is a mass of cells, the inner cell mass, attached eccentrically to the trophoblast Further changes are as follows

(a) Some cells of the inner cell mass differentiate (i.e. they become different from others) into flattened cells, that come to line its free surface (fig. 8A). These are called hypoblast and constitute the endoderm, which is thus the first germ layer to be formed.

(b) The remaining cell of the inner cell mass become columnar (fig. 8D). These are called are called epiblast and form the second germ layer, the ectoderm. The embryo is now in the form of a disc having two layers.

(c) A space appears between the ectoderm (below) and the trophoblast (above). This is the amniotic cavity (fig. 8C), filled by amniotic fluid, or liquor animal. The roof of his cavity is formed by amniogenic cells (cells of Rauber) derived from the trophoblast, while its floor is formed by the ectoderm.

(d) Flattened cells arising from the endoderm, spread and line the inside of the blastocystic cavity. In this way, a cavity, lined on all sides by cells of endodermal origin, is formed. This cavity is called the primary yolk sac (fig. 8D)

(e) The cells of the trophoblast give origin to a mass of cells called the extra-embryonic mesoderm (or primary mesoderm). These cells come to lie between the trophoblast and the flattened endodermal cells lining the yolk sac, thus separating them from each other. These cells also separate the wall of the amniotic cavity from the trophoblast (fig. 9A).

This mesoderm is called extra-embryonic because it lies out side the embryonic disc. It does not give rise to any tissues of the embryo itself.

(f) Small cavities appear in the extra-embryonic mesoderm. Gradually these join together to from larger spaces and, ultimately, on large space is formed. This cavity is called the extra-embryonic coelom (fig. 9B), it will be seen that the extra-embryonic coelom does not extend into that part of the extra-embryonic mesoderm which attaches the wall of the amniotic cavity to the trophoblast. The development embryo, along with the amniotic cavity and the yolk sac, is now suspended in the extra-embryonic coelom, and is attached to the wall of the blastocyst (i.e. trophoblast) only by this unsplitted part of the extra-embryonic mesoderm. This mesoderm forms a structure called the connecting stalk.

(g) Formation of chorion and amnion : At this stage, two very important membranes are formed. One is formed by the parietal extra-embryonic mesoderm (on the inside) and the overlaying trophoblast
(on the outside) this is called the chorian (fig. 9B). The other is the amnion which is constituted by the amniogenic cells forming the wall of the amniotic cavity (excluding the extodermal floor). These cells are derived from the trophoblast. We have already seen that the amnion is covered by the unsplit extra-embryonic mesoderm, and that the connecting stalk is attached to it.

(h) With the appearance of the extra-embryonic mesoderm, and later of the extra-embryonic coelom, the yolk sac becomes much smaller than before and is now called the secondary yolk sac. This alteration in size is accompanied by a change in the nature of the lining cells. They are no longer flattended but become cubical (fig. 9B).

(i) At this stage, the embryo proper is a circular disc composed of two layers of cells : the upper layer (towards amniotic cavity) is the ectoderm, the cells of which are columnar, while the lower layer (towards yolk sac) is the endoderm, made up of cubical cells (fig. 10)

(j) At one circular area a near the margin of the disc, the cubical cells of the endoderm become columnar.This area is called the prochordal plate. The appearance of the prochordal plate determines the central axis of the embryo (i.e. enables us to divide it into right and left halves), and also enables us to distinguish its head and tail ends (fig. 10)

(k) Soon after the formation of the prochordal plate some of the ectodermal cells lying along the central axis, near the tail end of the disc, begin to proliferate, and form an elevation that bulges into the amniotic cavity. This elevation is called the primitive streak (fig. 11). The primitive streak is at first a rounded or oval swelling primitive streak later forms the Henson's node.

(i) The cells that proliferate in the region of the primitive streak pass sideways, pushing themselves between the ectoderm and endoderm (fig. 11). These cells from the intra-embryonic mesoderm (or secondary mesoderm) which is the third germ layer.

The notochord is a midline structure, that develops from mesoderm in dorsal region.

Importance of the notochord

The notochord is present in all animals that belong to the phylum Chordata. In some of them e.g. Amphioxus, it persists into adult life and forms the central axis of the body. In others, including man, it appears in the embryo but only small remainants of it remain in the adult. Notochord elongates considerably, and lies in the midine, in the position to be position to be later occupied by the vertebral column. However, the notochord does not give rise to the vertebral column.

Neurulation : i.e formation of neural tube. After neurulation there occur three type of ectoderm.

(iii) Neural crest ectoderm.

Anterior part of neural tube differentiate brain and rest of the neural tube differentiate in spinal cord so central nervous system is formed by neural tube.

TERATOGENY :

During the first 3 months of pregnancy the basic structure of baby is formed. This involves cell division, cell migration, and differentiation of cells into the many types found in the body. During this period, the developing baby called foetus is very sensitive to anything that interferes with developmental steps. Eg.

Virus infection of mother by rubella (German measles) virus or exposure to certain chemicals may cause malformation in the developing embryo. Such agents are called teratogens (Monster Forming agents).

GENERAL STAGES OF EMBRYONIC DEVELOPMENT

1. Morula - As a result of segmentation or cleavage activities, unicellular zygote changes into a solid ball like multicellular structure. In the later stage of cleavage, clusters of sticky, cohering, protruding (otuside) blastomeres are produced, which look like mulberry. This stage is termed as morula stage.

2. Blastulation - Cleavage continues in solid ball like morula and new formed blastomeres start rearranging themselves. Cell-aggregation starts in blastomeres, due to the movement of these blastomeres a cavity appears in the embryo, it is called as blastocoel. This cavity is schizogenous cavity in origin i.e. it is formed by the separation of cells. Cell aggregation is also known as cohesion. Blastomeres arrange themselves in the form of a layer around the blastocoel, this layer is termed as blastoderm. The embryonic stage is now called blastula, and its formative activites is called blastulation.

Types of blastula

The shape of blastula depends on so many factors e.g. size of eggs, amount of yolk, distribution of yolk in the eggs, frequency of cleavage and number of cleavage divisions. According to these factors, we can classify blastula of different animals in different categories.

(e) Superficial blastula or Periblastula

(a) Coeloblastula - Blastocoel is wide and clear in this blastulation, it is completely surrounded by blastomeres on all the sides i.e, blastocoel cavity is situated totally inside the embryo. Blastomeres are very small in size as compared to blastocoel.

Example : - Eggs of Amphioxus, coelenterate, amphibia, sponges etc.

Coeloblastula of amphibians is called amphiblastula because in it blastocoel cavity is accentric in position and is more towards the animal pole (amount of yolk is more towards the vegetal pole).

Amphiblastula of sponges is a free swimming larva, its blastomeres are flagellated. This larva swims freely with the help of these flagella. It is a unique feature in sponges.

(b) Stereoblastula - In this blastula, blastocoel is very narrow or obliterated. Blastomeres are large as compared to blastocoel. It is almost solid.

Example - eggs of Neries, and members of phylum Mollusca.

(c) Discoblastula - It is found in those animals which have discoidal eggs. There is a cavity present in between blastomeres and yolk, it is called subgerminal cavity. It is a type of cavity, which is surrounded by yolk on one side and by blastomeres on the other side.

Examples - eggs of reptiles, birds and prototheria.

(d) Superficial blastula or periblastula - In centrolecithal eggs, cleavage occurs only in peripheral region. The layer of blastomeres surrounds the centrally situated yolk. Blastocoel is absent in this type of blastula. In place of blastocoel, subgerminal cavity is found.

Example - Eggs of insects.

(e) Blastocyst - blastula or Eutherian & Metahterian mammals is called blastocyst, because blastula is in the form of a cyst

Blastula - of mammals is called blastocyst. In blastocyst all the embryonal cells occur in the form of solid mass called embryonal knob. Embryonal knob (inner cell mass) is covered by protective layer called trophoblast and it's cell just above the embryonal knob are trophoblast called cells of Rauber (amniogenic cells). There occurs a cavity in between embryonal knob and trophoblast called albumin cavity. It is filled with nutritive fluid absorbed from the wall of uterus. So albumin cavity is also nutritive-cavity.

SPECIAL POINT

1. The growth phase is the longest phase during male gametogenesis. But in human oogenesis, maturation phase is longest.

2. The acrosome of sperm are produced by golgibodies.

3. The smallest sperm is of crocodile and its size of 0.02 mm & largest sperm is of Discoglossus (2 mm)

4. 74 days are required to complete the cycle of spermatogenesis in human being.

5. In 1 ml of semen, 20 to 120 millions of sperms are present in human being.

6. Deficiency in the number of sperms result in sterility which is known as oligospermia.

7. Absence of sperms in semen is known as azoospermia .

8. Formation of yolk in oogenesis takes place in the growth phase.

9. Largest egg is of Ostrich (16 cm long with its shell).

10. Although normal number of sperm are present in semen but if these are completely non motile. The condition is known as necrospermia.

11. Smallest egg in birds is of humming bird.

12. Due to high mortality rate in lower animals, the production of egg is more.

13. Sequence of egg production is as follows.

Mammals < Aves < Reptiles < Amphibian < Pisces.

14. Cat and rabbit both are induced ovulator.

15. The life span of eggs in female reproductive organs in human being is 48 hrs.

16. The nucleus of egg is known as germinal vesicle.

17. At the age of 45-50 yrs. In female the ovulation process will stop which is known as menopause.

18. The spermiation (release of sperms from sertoli cells) in all sertoli cells occurs simultaneously.

19. Cortical granules are absent in rat.

20. Mosaic type of cleavage is found in the parasite Echinococcus granulosus.

Special features of some animals :

(a) Sperms of some animals are not having flagella :

(1) Ascaris - sperm is amoeboid

(2) Cray fish - star shaped, tail less sperm

(3) In crab and lobuster the sperm are tail less and have three sharp process.

eg. In toad fish (Opsansus) head of many sperms unite together and form sperm boats

In Gastropods, the sperms are hexaflagellated.

Smallest sperm &ndash Crocodile (0.02 mm)

Largest sperm &ndash Discoglossus (2 mm) in chordates and Drosophila in entire animal kingdom.

Shape of head part of sperms :

(i) Spherical &ndash eg Teleostei

(ii) Lance shaped &ndash eg Amphibia and Reptiles

(iv) Spoon shaped &ndash eg. Mammals (in man)

Germinal layers and their derivatives

The following description gives an account of the respective organs formed by the three germ layers. Most of the organs are the product of combination of more than one germ layers.

Organs derived from ectoderm

1. Skin (epidermis) and their pigment cells.

2. Mucosal membrane of lips, cheek gums, basal portion of mouth, some part of palate, nasal apertures.

3. Lower part of anal canal.

5. Labia majora and outer part of labia minora.

6. Anterior epithelium of cornea, epithelium of conjunctiva, ciliary body and iris of eyes.

7. Outer face of tympanic membrane, epithelium of labyrinth.

9. Hairs, nails, enamel of teeth

Derivatives of mesoderm

1. Connective tissues, superficial and deep fascia, ligaments, tendons, dermis of skin. (from dermatome)

2. Specialized connective tissues like adipose tissue, reticular tissues, bones, cartilages.

5. Heart, all blood vessels and blood cells.

6. Kidneys, ureters, urinary bladder, posterior urethra of female, upper glandular part of prostate.

8. Testes, epididymis, vas defeens and seminal vesicle, ejaculatory duct.

9. Pleural cavities, peritoneal cavity and pericardial cavity.

11. Cornea, sclera, choroid ciliary body and iris related material.

12. Microglia, duramater etc.

Derivatives of endoderm

1. Epithelial part of mouth, some part of palate, tongue, tonsils, pharynx, oesophagus, stomach, small
and large intestine, upper part of anal canal.

2. Pharyngo-tympanic tube, middle ear, inner face of tympanic membrane.

4. Gall bladder, pancreatic duct.

5. Major protion of urinary bladder, complete urethra of female except posterior part, complete urethra
of male except anterior and posterior part.

6. Whole inner part of vagina including inner face of labia minora.

In addition to the above, the glands of gastrointestinal tract, major part of prostate etc. are also formed by endoderm.

Implantation

The attachment of developing embryo to the appropriate body layer or surface to obtain nutrition is called implantation. This phenomenon is a common event in most mammals (except prototheria) in which embryo (blastocyst stage) after reaching in uterus attaches itself with the wall of the uterus. In other animals like fishes, reptiles, birds, prototherian mammals etc., this nutritive connection is established with the yolk present in egg. In higher mammals including men, the blastocyst on its contact with endometrium of uterus gets completely buried in the wall of the uterus.

Initially the oocyte after its release from ovary, comes into fallopian tube where the process of fertilization is completed, Just after fertilization, embryonic development starts and a blastocyst is formed after cleavage and morulation. In human being, the blastocyst gets attached with the uterine endomdetrium in about four days after entering in uterus. At the same time, the cells of endometrium of implantation area separate out and adhere with embryonic cells with the help of certain enzymes secreted by the cells of trophoblast. In human, the site of implantation is generally mid-dorsal or mid-ventral part of uterus. Implantation of blastocyst takes about 7-8 days after fertilization in human and by 12 th day it is completely buried in the wall of the uterus. The place of entry through which the embryo enters into the wall, is completely closed by a fibrous and cellular plug, known as closing coagulum.

Types of Implantation

On the basis of the position of attachment in the uterus, implantation is of three types -

1. Central or Superficial implantation - In this type the blastocyst attaches superficially with the wall of uterus, and remains suspended in the lumen of the uterus. This type of implantation occurs in lower chordates, e.g. cow, pig, dog etc.

2. Interstitial implantation &ndash The blastocyst is buries deeply inside the wall of uterus and covered by endometrial tissues lying under epithelium. This type of implantation occurs in human being.

3. Eccentric implantation &ndash It occur in rat, squirrel etc. In this type of implantation , the blastocyst settles in the flods of epithelium of uterus. After some time it is completely surrounded by these folds.

9 months &ndash placenta attains maximum size, nails on fingers appear. In the next 10 days the foetus is ready to born as a little bady.

The above mentioned timing are approximate time periods. Some times, due to some reasons, certain babies are born before stipulated time. The babies born in 7 th month may also survive as normal babies.

EXTRA EMBRYONIC MEMBRANES AND PLACENTA

Extra embryonic membranes

In chordates like reptiles, birds and prototherian mammals, shaped blastula is a disc structure called as blastodis. The cellular layer formed of blastomeres remains as blastoderm. The central part of blastoderm gives rise to embryo proper, while the peripheral portion does not take past in the formation of embryo. This peripheral part is known as extra embryonic region. This region takes part in the formation of certain membranes called extra embryonic membranes. These extra embryonic membranes provide facilities for nutrition, respiration and excretion to the embryo. Extra embryonic membranes are of four types &ndash

On the basis of presence of absence of amnion, two groups of vertebrates are categorised.

1. Amniota - This group is characterized with the presence of amnion in the embryos of its members. For example members of class Reptilia, Aves and Mammalia.

2. Anamniota - Animals of this group are devoid of amnion in their embryos. For example class cyclostomata, pisces and amphibia.

Extra embryonic membranes in human

The process of gastrulation in embryo results into the formation of endoderm or hypoblast, ectoderm or epiblast, amniotic cavity, yolk sac, extra embryonic parietal and visceral mesoderm, connecting stalk etc. Extra embryonic membranes are also formed during this process. Each extra embryonic membrane is derived from two layers.

1. Amnion - It is formed by the layer of amniogenic cells present around the amniotic cavity and the extra embryonic mesoderm. Extra embryonic mesoderm layer surrounds the amnion. The connecting stalk is also attached with it. With a gradual increase in size the amnion covers the embryo from all sides. After about eight weeks of fertilization, amnion is completely incorporated into connecting stalk, which finally forms the umbilical cord. Embryo, in this stage, is called as foetus remains hanging in amniotic fluid.

2. Chorion - It is formed by the extra embryonic parietal layer of mesoderm and the cell of trophoblast. After implantation of blastocyst, the trophoblast gives out several figner like processes, the chorionic villi which get embedded into uterine endometrium Mesoderm also contributes in the formation of these villi. After a period of four these villi disappear from all parts except the connecting stalk where they grow rapidly and participate in the formation of placenta.

3. Yolk sac - Yolk sac is formed by the cells of extra embryonic visceral mesoderm and endoderm. Initially the sie of yolk sac is larger as compared to that of the embryo. About eight weeks after fertilization, the yolk is reduced in size and changes into a tubular structure. Ultimately a placenta is developed with the incorporation of yolk sac and mesodermal connecting stalk with the amnion and chorion.

4. Allantois - It is a solid and cylindrical mass formed by embryonic mesoderm. A small cavity lined by endodermal cells develops in it. The mesoderm of allantois forms many small blood vessels in this region. These vessels connect the embryo with placenta and ensure nutritional and respiratory supply to embryo. In human, allantois does not function to store the excretory wastes as it does in reptiles and birds.

The eggs of viviparous animals are unable to develop into their embryos outside the uterus independently. This is because of the very little or negligible amount of yolk present in these eggs, which can not fulfill the nutritional and other physiological demands of a developing embryo. Here the embryo depends upon maternal tissues for shelter, nutrition, respiration etc. These animals therefore, have developed adaptation, respiratory and other physiological requirements from mother's body.

Placenta is found in all viviparous (exept sub-class-prototheria oviparous) animals.

Structure of Placenta

Placenta is not a simple membrane. It is made up of the tissues from two different sources &ndash

1. Maternal tissue - These include uterine epithelium, connective tissues and blood capillaries.

2. Embryonic tissue - These include extra embryonic membranes (mainly chorion). Yolk sac and allantois may also take part in placenta formation.

Embryonic connective tissues and blood capillaries are also constituents of it. On the basis of extra embryonic membranes, the placenta is of three types.

1. Yolk sac placenta - It is formed by yolk sac and uterine epithelium. For example, Elasmobrancs (Sharks), Mustelus etc.

2. Choria-vitelline placenta - It is formed by chorion and yolk sac combinely. Hence it is called as choriovitelline placenta. For example, Didelphis, Macropus and other metatherian mammals.

3. Chorio-allantoic placenta - This type of placenta is formed by embryonic chorion and allantoic membranes. It is also referred to as a true placenta. It is found in eutherian mammals.

Chorio- allantoic placenta in mammals.

1. In this type of placenta, allantoic mesodern and the mesoderm of umbilical cord jointly form the blood vessels of umbilical cord. The endodermal part of the allantois remains as a very small cavity.

2. To obtain nutrition from maternal blood several finger like processes or villi are formed by chorion which penetrate deeply into the crypts of uterus. Initially the villi are scattered over the whole surface of chorion but later they become restricted in the deciduas besalis region. The chorionic villi on the remaining surface disappear shortly. The part of chorion, which helps in placenta formation is known as chorionic frondosum.

Classification of Placenta

On the basis of different characters, the placenta are classified in following manner &ndash

1. On the basis intimacy

After implantation, the wall of uterus is called as deciduas, instead of endometrium. The part of

deciduas, where placenta is formed is called deciduas basalis whereas, the part separating the embryo from lumen of uterus is called deciduas capsularis. The remaining part of lumen of uterus is called deciduas parietalis. Decidua also comes out from uterus at the time of parturition. On the basis of intimacy between embryo and uterine wall the placenta is classified into three classes &ndash

(i) Non-deciduate or Semi placenta - In this type of placenta, there is no close and rigid association between embryo and the wall of uterus. Hence, at the time of parturition, there is no bleeding as the chorionic villi are easily pulled out from the crypts of uterus. For example, cow, buffalo, horse, pig.

(ii) Contra-deciduate placenta - There is a close association between embryonic and maternal tissues. However at parturition, the damaged maternal and embryonic tissues along with the part of placenta remain inside the uterus which are absorbed is situ by leucocytes. For example &ndash Parameles, Talpa etc.

(iii) Deciduate placenta - This type of placenta is found in human, dog, hare etc. It is characterized with a very close association between chorionic villi and uterine wall. At the time of birth, the mucosal covering of the uterus is also damaged and discarded outside. This results in an extensive bleeding at child birth. This placenta is known as a true placenta.

2. On the basis of implantation

Three types of placenta are found on the basis of implantation.

(i) Superficial - When the placenta is situated in the lumen of uterus. For example, Parameles, pig, cow,
cat etc.

(ii) Eccentric - The placenta is situated in the fold or pocket of the cavity of uterus. For example - rat,
squirrel etc.

(iii) Interstitial &ndash This type of placenta is found in man, guinea pig, apes etc. The chorionic sac (placenta) penetrates deep inside the wall of uterus. Hence, the association between embryo and maternal part becomes very close.

3. On the basis of distribution of villi

On this basis, the placenta are of four types.

(i) Diffused placenta &ndash The villi are scattered on the whole surface of placenta. For example pig, horse, lemur etc.

(ii) Cotyledonary placenta &ndash The villi are distributed in small isolated groups on the chorionic surface. These groups of villi are called as cotyledons. For example, cow, buffalo, sheep, deer etc.

(iii) Zonary placenta - This type of placenta have the villi distributed in a belt shaped zone which is large sized and circular.

Zonary placenta is of two types &ndash

(a) Complete zonary placenta - The belt of villi is complete and ring shaped in it. For example - dog, cat, lion etc.

(b) Incomplete Zonary placenta - The belt of villi is incomplete in it. For example - raccoon.

(iv) Discoidal placenta - In this type of placenta, whole of the chorionic surface is covered by villi in initial stage, but the villi disappear later from later from most area except the region of implantation, that is only a disc like region is left with villi. Discoidal placenta is also of two types &ndash

(a) Mono discoidal placenta - The villi are present only on dorsal surface in single circular disc like area. For example &ndash human, hare etc.

(b) Bi discoidal placenta - If the villi are distributed in two disc like areas, the placenta is called as bidiscoidal, eg. Monkeys.

4. On the basis of histology

The blood of maternal and embryonic do not mix together through placenta. The blood circulations of the two sides are kept separated by one or more layers described below &ndash

The transportation of various materials takes place by diffusion through these six layers the intimacy between maternal and embryonic tissues in different mammals is determined by the presence or absence of these layers in placenta. Therefore, on the basis of presence or absence of the above layers, the placenta is of five types.

1. Epitheliochorial - It is the most primitive type of placenta in which all the six layers. mentioned
earlier, remain intact. For example pig, horse etc.

2. Syndesmochorial - In this type of placenta the uterine epithelium is eroded by chorionic villi, so only two maternal layers remain functional. Therefore, along with three foetal layers, total five layers are preent in this placenta. For example - sheep, goat, cow etc.

3. Endotheliochorial - Here uterine connective tissue layer is also damaged along with uterine epithelium layer. Therefore only four layers (3 foetal and one maternal) are found in this placenta eg. dog, cat, etc.

4. Haemochorial &ndash All the three maternal layers are penetrated in this placenta. The chorionic epithelium comes in direct contact with uterine blood sinusoids. For example &ndash man, monkey, bat etc.

5. Haemoendotheliochorial/Haemoendothelial : It is the most typical placental in which the trophoblastic epithelium of embryo is also eroded along with all three maternal layers. The foetal capillaries are in direct contact with maternal blood. For example rat, guinea pig, rabbit etc.

The placenta of human mainly secretes two steroid hormones like estradiol and progesterone, and two protein hormones like human chorional gonadotropin HCG and human placental somatomammotropin HCS large amount of &ndashHCG, hormone is secreted, during early pregnancy, from the placenta. Because of this reason its quantity increases in the urine of pregnant lady. On the basis of this fact, pregnancy test is performed. The above hormones are also held responsible for keeping the corpus luteum active, protection of embryo, prevention of abortion and growth of mammary glands.

Functions of placenta

1. Exchange of important materials between foetal and maternal blood.

2. The essential materials are exchanged by diffusion, pinocytosis or active transport.

3. The small molecules like O2, CO2, H2O etc. and other inorganic substances like chlorides, phosphates, sodium, potassium, magnesium etc. are also diffused through placenta.

4. Large molecules like lipids, polysaccharides, carbohydrates proteins etc. are obtained by pinocytosis process.

5. The nutritional substances are supplied to embryo from the mother through placenta.

6. Placenta also serves as a respiratory medium for exchange of O2 and CO2 between embryo and mother.

7. The nitrogenous and metabolic wastes from foetus are released into the blood of mother by diffusion through placenta.

8. The antibodies for measles, chickenpox, polio etc. present in the blood of mother reach the embryo through placenta.

9. Pathogenic viruses may also enter in embryo through placenta.

10. If a female takes some harmful chemicals, liquor, drugs etc. during pregnancy, these may cross the placenta and on reaching into foetus may cause deformity during organogenesis. (eg. Thallidomide)

11. Placenta itself secretes some hormones like progesterone, estrogen, lactogen, HCG, HCS etc.

12. Progesterone, maintains and supports the foetus during the whole pregnancy period. At the time of parturition, relaxin is secreted by placenta which lubricates and widens the birth canal to facilitate child birth.

REPRODUCTIVE HEALTH

Methods of contraception

An ideal contraceptive should be user-friendly, easily available, effective and reversible with no or least side-effects. It also should in no way interfere with the sexual drive, desire and/or the sexual act of the user. A wide range of contraceptive methods are presently available which could be broadly grouped into the following categories, namely Natural/Traditional, Barrier, IUDs, Oral contraceptives, Injectables, Implants and Surgical methods.

(1) Natural methods :

work on the principle of avoiding chances of ovum and sperms meeting. Periodic abstinence is one such method in which the couples avoid or abstain from coitus from day 10 to 17 of the menstrual cycle when ovulation could be expected. As chances of fertilisation are very high during this period, it is called the fertile period. Therefore, by abstaining from coitus during this period, conception could be prevented.

Withdrawal or coitus interruptus is another method in which the male partner withdraws his penis from the vagina just before ejaculation so as to avoid insemination.

Lactational amenorrhea (absence of menstruation) method is based on the fact that ovulation and therefore the cycle do not occur during the period of intense lactation following parturition. Therefore, as long as the mother breast-feeds the child fully, chances of conception are almost nil. However, this method has been reported to be effective only upto a maximum period of six months following parturition. As no medicines or devices are used in these methods, side effects are almost nil. Chances of failure, though, of this method are also high.

(2) Barrier Methods :-

In barrier methods, ovum and sperms are prevented from physically meeting with the help of barriers. Such methods are available for both males and females. Condoms are barriers made of thin rubber/latex sheath that are used to cover the penis in the male or vagina and cervix in the female, just before coitus so that the ejaculated semen would not enter into the female reproductive tract. This can prevent conception. &lsquoNirodh&rsquo is a popular brand of condom for the male. Use of condoms has increased in recent years due to its additional benefit of protecting the user from contracting STDs and AIDS. Both the male and the female condoms are disposable, can be self-inserted and thereby gives privacy to the user.

Diaphragms, cervical caps and vaults are also barriers made of rubber that are inserted into the female reproductive tract to cover the cervix during coitus. They prevent conception by blocking the entry of sperms through the cervix. They are reusable.

Spermicidal creams, jellies and foams are usually used alongwith these barriers to increase their contraceptive efficiency.

(3) Intra Uterine Devices (IUDs) :

These devices are inserted by doctors or expert nurses in the uterus through vagina. These Intra Uterine Devices are presently available as the non-medicated IUDs (e.g., Lippes loop), copper releasing IUDs (CuT, Cu7, Multiload 375) and the hormone releasing IUDs (Progestasert, LNG-20).

IUDs increase phagocytosis of sperms within the uterus and the Cu ions released suppress sperm motility and the fertilising capacity of sperms.

The hormone releasing IUDs, in addition, make the uterus unsuitable for implantation and the cervix hostile to the sperms.

IUDs are ideal contraceptives for the females who want to delay pregnancy and/or space children. It is one of most widely accepted methods of contraception in India.

(4) Oral contraceptives :

Oral administration of small doses of either progestogens or progestogen&ndashestrogen combinations is another contraceptive method used by the females. They are used in the form of tablets and hence are popularly called the pills. Pills have to be taken daily for a period of 21 days starting preferably within the first five days of menstrual cycle. After a gap of 7 days (during which menstruation occurs) it has to be repeated in the same pattern till the female desires to prevent conception.

They inhibit (primarily) ovulation and implantation as well as they alter the quality of cervical mucus to prevent/retard entry of sperms. Pills are very effective with lesser side effects and are well accepted by the females.

Saheli &ndash the new oral contraceptive for the females contains a non-steroidal preparation. It is a &lsquoonce a week&rsquo pill with very few side effects and high contraceptive value. It was developed by scientists at Central Drug Research Institute (CDRI) in Lucknow.

Progestogens alone or in combination with estrogen can also be used by females as injections or implants under the skin Their mode of action is similar to that of pills and their effective periods are much longer.

Administration of progestogens or progestogen-estrogen combinations or IUDs within 72 hours of coitus have been found to be very effective as emergency contraceptives as they could be used to avoid possible pregnancy due to rape or casual unprotected intercourse. This is termed as emergency contraception

(5) Surgical methods :

Also called sterilisation, are generally advised for the male/female partner as a terminal method to prevent any more pregnancies. Surgical intervention blocks gamete transport and thereby prevent conception. Sterilisation procedure in the male is called &lsquovasectomy&rsquo and that in the female, &lsquotubectomy&rsquo.

In vasectomy, a small part of the vas deferens is removed or tied up through a small incision on the scrotum whereas in tubectomy, a small part of the fallopian tube is removed or tied up through a small incision in the abdomen or through vagina. These techniques are highly effective but their reversibility is very poor.

Medical Termination of Pregnancy (MTP)

Intentional or voluntary termination of pregnancy before full term is called medical termination of pregnancy (MTP) or induced abortion. Nearly 45 to 50 million MTPs are performed in a year all over the world which accounts to 1/5th of the total number of conceived pregnancies in a year. Obviously, MTP has a significant role in decreasing the population though it is not meant for that purpose. Whether to accept / legalise MTP or not is being debated upon in many countries due to emotional, ethical, religious and social issues involved in it. Government of India legalised MTP in 1971 with some strict conditions to avoid its misuse. Such restrictions are all the more important to check indiscriminate and illegal female foeticides which are reported to be high in India.

MTP is performed to get rid of unwanted pregnancies either due to casual unprotected intercourse or failure of the contraceptive used during coitus or rapes. MTPs are also essential in certain cases where continuation of the pregnancy could be harmful or even fatal either to the mother or to the foetus or both. MTPs are considered relatively safe during the first trimester, i.e., upto 12 weeks of pregnancy. Second trimester abortions are much more riskier. One disturbing trend observed is that a majority of the MTPs are performed illegally by unqualified quacks which are not only unsafe but could be fatal too.

Another dangerous trend is the misuse of amniocentesis to determine the sex of the unborn child. Frequently, if the foetus is found to be female, it is followed by MTP- this is totally against what is legal. Such practices should be avoided because these are dangerous both for the young mother and the foetus. Effective counselling on the need to avoid unprotected coitus and the risk factors involved in illegal abortions as well as providing more health care facilities could reverse the mentioned unhealthy trend.

Sexually Transmitted Diseases (STDs)

Diseases or infections which are transmitted through sexual intercourse are collectively called sexually transmitted diseases (STD) or venereal diseases (VD) or reproductive tract infections (RTI). Gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B and of course, the most discussed infection in the recent years, HIV leading to AIDS are some of the common STDs. Among these, HIV infection is most dangerous

Some of these infections like hepatitis&ndashB and HIV can also be transmitted by sharing of injection needles, surgical instruments, etc., with infected persons, transfusion of blood, or from an infected mother to the foetus too.

Except for hepatitis-B, genital herpes and HIV infections, other diseases are completely curable if detected early and treated properly.

Early symptoms of most of these are minor and include itching, fluid discharge, slight pain, swellings, etc., in the genital region. Infected females may often be asymptomatic and hence, may remain undetected for long.

Absence or less significant symptoms in the early stages of infection and the social stigma attached to the STDs, deter the infected persons from going for timely detection and proper treatment. This could lead to complications later, which include pelvic inflammatory diseases (PID), abortions, still births, ectopic pregnancies, infertility or even cancer of the reproductive tract.

STDs are a major threat to a healthy society. Therefore, prevention or early detection and cure of these diseases are given prime consideration under the reproductive health-care programmes. Though all persons are vulnerable to these infections, their incidences are reported to be very high among persons in the age group of 15-24 years.

Preventive Measures :

(i) Avoid sex with unknown partners/multiple partners.

(ii) Always use condoms during coitus.

(iii) In case of doubt, go to a qualified doctor for early detection and get complete treatment if diagnosed with disease.

Infertility

A large number of couples all over the world including India are infertile, i.e., they are unable to produce children inspite of unprotected sexual co-habitation. The reasons for this could be many&ndashphysical, congenital, diseases, drugs, immunological or even psychological. In India, often the female is blamed for the couple being childless, but more often than not, the problem lies in the male partner. Specialised health care units (infertility clinics, etc.) could help in diagnosis and corrective treatment of some of these disorders and enable these couples to have children. However, where such corrections are not possible, the couples could be assisted to have children through certain special techniques commonly known as assisted reproductive technologies (ART).

Assisted reproductive technologies (ART)

In vitrofertilisation (IVF&ndashfertilisation outside the body in almost similar conditions as that in the body) followed by embryo transfer (ET) is one of such methods. In this method, popularly known as test tube baby programme, ova from the wife/donor (female) and sperms from the husband/donor (male) are collected and are induced to form zygote under simulated conditions in the laboratory. The zygote or early embryos (with upto 8 blastomeres) could then be transferred into the fallopian tube (ZIFT&ndashzygote intra fallopian transfer) and embryos with more than 8 blastomeres, into the uterus (IUT &ndash intra uterine transfer), to complete its further development.

Embryos formed by in-vivo fertilisation (fusion of gametes within the female) also could be used for such transfer to assist those females who cannot conceive. Transfer of an ovum collected from a donor into the fallopian tube (GIFT &ndash gamete intra fallopian transfer) of another female who cannot produce one, but can provide suitable environment for fertilisation and further development is another method attempted.

Intra cytoplasmic sperm injection (ICSI) is another specialised procedure to form an embryo in the laboratory in which a sperm is directly injected into the ovum.

Infertility cases either due to inability of the male partner to inseminate the female or due to very low sperm counts in the ejaculates, could be corrected by artificial insemination (AI) technique. In this technique, the semen collected either from the husband or a healthy donor is artificially introduced either into the vagina or into the uterus (IUI &ndash intra-uterine insemination) of the female.

General Information

In lower animals, as a result of asexual reproduction the structure formed is termed as blastema and it gives rise to the complete animal. This is termed as ''blastogenesis'' and the animals so formed are termed as blastozoids.

In majority of animals, as a result of sexual reproduction the structure formed is termed as the zygote and it gives rise to complete animal. This is termed as ''embryogenesis'' and the animals, so formed are termed as oozoids. They have the most advanced characters.

(1) Aristotle is known as ''Father of Embryology'' he first studied the development in chick and other embryos. He gave its description in his book ''Historia Animalia''.

(2) Leeuvenhock (1671) - He observed and described human sperm for the first time.According to Hartsoeker and Leeuvenhock there is a small model of developing animals present in the head of the sperm of that animal. This small model is called homunculus. Both these scientists are called spermists, and this theory is called '' Theory of spermist ''.

(3) Swammer Dame, Haller, Bonette & Malpighi : - According to these scientists, small model of animal is always present in the egg. These scientists are called Ovists, and their theory is known as 'Ovists theory'

(4) Schleiden & Schwann : - Both the scientists established the cellular structure of egg and sperm.

(5) Pander : - He described the presence of three germinal layers in chick embryo.

(6) Fredrich Wolff : - He first presented the ''theory of epigenesis''.

(7) Muller : - He gave the recapitulation theory.

(8) Haeckel : - He gave the details of Recapitulation theory and named it as the bio-genetic law.

- Bio-genetic Law : - According to this each organism during its embryonal development, passes through all stages, through which its species has evolved or embryo repeats its ancestry. i.e. Ontogeny recapitulates its Phylogeny.

(9) ''Carl Ernest Von Baer'' : - He is known as the ''father of modern embryology.'' He gave the Baer's Law which in turn proves the recapitulation theory.

According to this law, during embryonal development, the development of general structures takes place earlier and specific structures develop at last or later on.

(10) A. Weismann : - He gave the theory of germplasm or the theory of continuity of germplasm. According to him, there are 2 types of protoplasm in the body of animals : -

Somatoplasm dies but the germplasm is never destroyed, rather it is transferred to the progenies.

(11) Wilhelim : - He studied embryonal development in frog and gave the mosaic theory.

- He said that there are some presumptive areas in the eggs of frog. These areas form specific structures during embryonal development. This is termed as ''Promorphology .'' and these type of eggs are termed as the mosaic eggs.

(12) Hand Driesch : - He studied embryonal development is sea-urchin and gave the regulative theory.

- In the eggs of Sea-Urchin presumptive areas are not found i.e. promorphology is not found. So, each part of the egg is capable of forming the complete embryo. These type of eggs are termed as regulative eggs.

(13) Boveri & Child : -

They gave the gradient theory to explain the mosaic development in eggs.

According to them, a metabolic gradient is present inside the eggs.

Different parts of the egg have different metabolic rates.

The rate of metabolism is faster at the animal-pole of the egg and is slower at the vegital pole of the egg.

(14) Spemann :- He gave the ''Theory of organizers''.

- According to it embryo has some special type of tissues, which induce development of some specific structures.

These are termed as the organizers.

- These organizers secrete some special chemicals called evocators which induce the formation of

- Spemann got the Nobel prize for his theory of organizers.

(15) R.V. Graff : -

- He studied a follicle in human ovary and termed it as ''Graafian follicle''


Quadrant Model of Reality

I'm a paragraph. Click here to add your own text and edit me. It's easy.

I'm a paragraph. Click here to add your own text and edit me. It's easy.

Fåhræus (a Swedish physician who devised the erythrocyte sedimentation rate) suggested that the Ancient Greek system of humorism, wherein the body was thought to contain four distinct bodily fluids (associated with different temperaments), were based upon the observation of blood clotting in a transparent container. When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen. A dark clot forms at the bottom (the "black bile"). Above the clot is a layer of red blood cells (the "blood"). Above this is a whitish layer of white blood cells (the "phlegm"). The top layer is clear yellow serum (the "yellow bile").[33]

The ABO blood group system was discovered in the year 1900 by Karl Landsteiner. Jan Janský is credited with the first classification of blood into the four types (A, B, AB, and O) in 1907, which remains in use today. In 1907 the first blood transfusion was performed that used the ABO system to predict compatibility.[34] The first non-direct transfusion was performed on March 27, 1914. The Rhesus factor was discovered in 1937.

Jan Janský also discovered the human blood groups in 1907 which he classified blood into four groups I, II, III, IV. Titled in Czech "Hematologická studie u psychotiků". His nomenclature is still used in Russia and states of the former USSR, in which blood types O, A, B, and AB are respectively designated I, II, III, and IV.

IT IS WAS ARGUED HE DIED AT 256 YEARS OLD------- 256 IS FOUR TO THE FOURTH POWER

Li Ching-Yuen or Li Ching-Yun (simplified Chinese: 李清云 traditional Chinese: 李清雲 pinyin: Lǐ Qīngyún) (claimed to be born 1677 or 1736 - died 6 May 1933) was a Chinese herbalist, martial artist and tactical advisor, known for his supposed extreme longevity.[4][5] He claimed to be born in 1736, while disputed records suggest 1677. Both claimed lifespans of 197 and 256 years, far exceeding the longest confirmed lifespan of 122 years and 164 days of the French woman Jeanne Calment. His true date of birth was never determined and his claims have been dismissed by gerontologists as a myth.[6]

THE FRUIT IS CROSS SHAPED
https://en.wikipedia.org/wiki/Plagiobothrys_shastensis
The fruit is a cross-shaped nutlet 2 or 3 millimeters wide divided in half by a rough scar.

Tetralogy of Fallot (TOF) is a congenital heart defect that is present at birth.[2] Symptoms include episodes of bluish color to the skin. When affected babies cry or have a bowel movement, they may develop a "tet spell" where they turn very blue, have difficulty breathing, become limp, and occasionally lose consciousness. Other symptoms may include a heart murmur, finger clubbing, and easy tiring upon breastfeeding.[3]

The cause is typically not known. Risk factors include a mother who uses alcohol, has diabetes, is over the age of 40, or gets rubella during pregnancy. It may also be associated with Down syndrome.[4] Classically there are four defects:[2]

a ventricular septal defect, a hole between the two ventricles

pulmonary stenosis, narrowing of the exit from the right ventricle

right ventricular hypertrophy, enlargement of the right ventricle

an overriding aorta, which allows blood from both ventricles to enter the aorta

Rudolphina Menzel, an immigrant to Mandate Palestine from Austria, having studied the desert free living dogs and the variations in appearances, classified these canines into four types: 1) heavy, sheepdog appearance, 2) dingo-like appearance, 3) Border Collie appearance, 4) Greyhound appearance. Menzel concluded that the Canaan dog is a derivative of the Type III pariah &mdash the collie type (referring to the type of farm collie found in the 1930s which was a medium dog of moderate head type more similar to today's Border Collie, not the modern Rough Collie).

Pavlov extended the definitions of the four temperament types under study at the time: phlegmatic, choleric, sanguine, and melancholic, updating the names to "the strong and impetuous type, the strong equilibrated and quiet type, the strong equilibrated and lively type, and the weak type."

THE MOST SUCCESS IS WITH FOUR BREEDS OF DOGS- FOUR MAINLY GRADUATED

In a recent study that was done, there was a noticeable amount of successes with mostly 4 breeds of dogs. The four that were mainly graduated and able to become service dogs were Labrador retrievers, golden retrievers, German shepherds, and Labrador/Golden Retriever crosses. In the study, they found that the Labrador/golden retriever crosses and the Labrador retrievers were the most successful to train in a shorter period of time. Whereas German shepherds and Golden retrievers were more successful if they were trained for a longer period of time instead of a 4-month period.[24]

THE FOUR QUADRANTS OF OPERANT CONDITIONING

FOUR LEARNING QUADRANTS PAVLOV

Now there are actually four learning quadrants that can be used as &ldquoconsequences&rdquo for behaviour so let&rsquos take a look at them. learningquadrantWhen the aim is to change behaviour we can try to decrease the frequency of the behaviour or increase the frequency of the behaviour. To accomplish the goal we can either add a stimulus or remove a stimulus. Here are some common dog training examples of each.

Positive Reinforcement to increase the frequency of a behaviour by adding a stimulus a common example is giving a treat to your dog when she sits, which increases the likelihood that she will sit again by adding the food treat.

Negative Reinforcement to increase the frequency of a behaviour by removing a stimulus a common example is removing something the dog doesn&rsquot like to get compliance like holding the collar tight on the dog&rsquos throat until the dog sits.

Positive Punishment to decrease the frequency of a behaviour by adding a stimulus a common example is if the dog is pulling on leash, the owner pops the leash causing the dog discomfort. Some training tools that are based on positive punishment learning theory are Spray Bottles, Slip Collars (choke chains), Pinch Collars, and Electronic Shock Collars.

Negative Punishment to decrease the frequency of a behaviour by removing a stimulus a common example is removing attention or freedom from a dog who is behaving in an unruly manner (like a time out).

In force free learning theory, like the training that Lisi and I teach, we rely heavily on Positive Reinforcement and Negative Punishment to motivate dogs to learn and repeat behaviours that we want to see in real life. We have found through experiences with many different dogs that we get the best results, and feel the best about our training methods when we don&rsquot need to use intimidation and force heavy methods to get the behaviours that we want

The genus Rosa is subdivided into four subgenera:

Hulthemia (formerly Simplicifoliae, meaning "with single leaves") containing one or two species from southwest Asia, R. persica and Rosa berberifolia which are the only roses without compound leaves or stipules.

Hesperrhodos (from the Greek for "western rose") contains Rosa minutifolia and Rosa stellata, from North America.

Platyrhodon (from the Greek for "flaky rose", referring to flaky bark) with one species from east Asia, Rosa roxburghii (also known as the chestnut rose).

Rosa (the type subgenus, sometimes incorrectly called Eurosa) containing all the other roses. This subgenus is subdivided into 11 sections.

The flowers of most species have five petals, with the exception of Rosa sericea, which usually has only four. Each petal is divided into two distinct lobes and is usually white or pink, though in a few species yellow or red. Beneath the petals are five sepals (or in the case of some Rosa sericea, four). These may be long enough to be visible when viewed from above and appear as green points alternating with the rounded petals. There are multiple superior ovaries that develop into achenes.[4] Roses are insect-pollinated in nature.

THERE ARE FOUR FORMAE OF THE FOUR LEAF ROSE

Rosa sericea, the silky rose,[1] is a species of Rosa native to southwestern China (Guizhou, Sichuan, Xizang, Yunnan), Bhutan, northern India (Sikkim), Nepal and Myanmar it grows in mountains at altitudes of 2,000-4,400 m.

It is a shrub growing to 2 m tall, often very spiny. The leaves are deciduous, 4&ndash8 cm long, with 7&ndash11 leaflets with a serrated margin. The flowers are 2.5&ndash5 cm diameter, white, with (unusually for a rose) only four petals. The hips are red, 8&ndash15 mm diameter, with persistent sepals, and often bristly.

Rosa sericea f. glandulosa T.T.Yü & T.C.Ku.

Rosa sericea f. glabrescens Franchet.

Rosa sericea f. pteracantha Franchet.

FOUR FORMAE AND FOUR PETALS ROSE

Rosa omeiensis is a species of Rosa native to central and southwestern China in the provinces of Gansu, Guizhou, Hubei, Ningxia, Qinghai, Shaanxi, Sichuan, Xizang, and Yunnan it grows in mountains at altitudes of 700 to 4,400 m.

It is a shrub growing to 4 m tall, often very spiny. The leaves are deciduous, 3&ndash6 cm long, with 5-13 leaflets with a serrated margin. The flowers are 2.5-3.5 cm diameter, white, with (unusually for a rose) only four petals. The hips are red to orange-yellow, 8&ndash15 mm diameter, with persistent sepals, and often bristly.

Rosa omeiensis f. omeiensis.

Rosa omeiensis f. glandulosa T.T.Yü & T.C.Ku.

Rosa omeiensis f. paucijuga T.T.Yü & T.C.Ku.

Rosa omeiensis f. pteracantha Rehder & E.H.Wilson.

It is sometimes treated as a subspecies of the closely related species Rosa sericea.

Four basic parts[change | change source]

Flowers have four basic parts, from the outside in they are:

The perianth, the vegetative parts

The calyx: the outermost whorl consisting of units called sepals. These are often green and enclose the rest of the flower in the bud. They may be absent, or they may be petal-like in some species.

The corolla: the petals, usually thin, soft and often colored to attract animals that help pollination.

The androecium, the male part, is the stamens

The gynoecium, the female parts,

The plentiful, fragrant flowers are produced in large, showy, terminal racemes that can be 30+ cm tall and elongate as the flowers of the inflorescence bloom. When stems have both flowers and fruits, the weight sometimes causes the stems to bend. Each flower is large (2 cm across), with four petals. Flower coloration varies, with different shades of lavender and purple most common, but white, pink, and even some flowers with mixed colors exist in cultivated forms. A few different double-flowered varieties also exist.[3] The four petals are clawed and hairless. The flowers have six stamens in two groups, the four closest to the ovary are longer than the two oppositely positioned. Stigmas are two-lobed. The four sepals are erect and form a mock tube around the claws of the petals and are also colored similarly to the petals.[4]

Some plants may bloom until August, but warm weather greatly shortens the duration on each flower's blooming. Seeds are produced in thin fruits 5&ndash14 cm long pods, containing two rows of seeds separated by a dimple. The fruit are terete and open by way of glabrous valves, constricted between the seeds like a pea pod. Seeds are oblong, 3&ndash4 mm long and 1&ndash1.5 mm wide.[5]

In North America, Hesperis matronalis is often confused with native Phlox species that also have similar large showy flower clusters. They can be distinguished from each other by foliage and flower differences: dame's rocket has alternately arranged leaves and four petals per flower, while phloxes have opposite leaves and five petals.

FOUR PARTS TREE
https://simple.wikipedia.org/wiki/Tree
A tree is a tall plant with a trunk and branches made of wood. Trees can live for many years. The oldest tree ever discovered is approximately 5,000 years old. The four main parts of a tree are the roots, the trunk, the branches, and the leaves.

THE FOURTH IS ALWAYS DIFFERENT

Traditionally, several Native American groups planted sunflowers on the north edges of their gardens as a "fourth sister" to the better known three sisters combination of corn, beans, and squash.[9]

It is noteworthy that the four subfamilies Asteroideae, Cichorioideae, Carduoideae and Mutisioideae contain 99% of the species diversity of the whole family (approximately 70%, 14%, 11% and 3% respectively).

LOOK AT THE REPETITION OF QUADRANT AND QUARTET- THIS I SALL ALREADY POSTED IN MY OVER 60 QMR BOOKS
https://en.wikipedia.org/wiki/Cleavage_(embryo)
Spiral cleavage is conserved between many members of the lophotrochozoan taxa, referred to as Spiralia.[3] Most spiralians undergo equal spiral cleavage, although some undergo unequal cleavage (see below).[4] This group includes annelids, molluscs, and sipuncula. Spiral cleavage can vary between species, but generally the first two cell divisions result in four macromeres, also called blastomeres, (A, B, C, D) each representing one quadrant of the embryo. These first two cleavages are oriented in planes that occur at right angles parallel to the animal-vegetal axis of the zygote.[3] At the 4-cell stage, the A and C macromeres meet at the animal pole, creating the animal cross-furrow, while the B and D macromeres meet at the vegetal pole, creating the vegetal cross-furrow.[5] With each successive cleavage cycle, the macromeres give rise to quartets of smaller micromeres at the animal pole.[6][7] The divisions that produce these quartets occur at an oblique angle, an angle that is not a multiple of 90°, to the animal-vegetal axis.[7] Each quartet of micromeres is rotated relative to their parent macromere, and the chirality of this rotation differs between odd and even numbered quartets, meaning that there is alternating symmetry between the odd and even quartets.[3] In other words, the orientation of divisions that produces each quartet alternates between being clockwise and counterclockwise with respect to the animal pole.[7] The alternating cleavage pattern that occurs as the quartets are generated produces quartets of micromeres that reside in the cleavage furrows of the four macromeres.[5] When viewed from the animal pole, this arrangement of cells displays a spiral pattern.

D quadrant specification through equal and unequal cleavage mechanisms. At the 4-cell stage of equal cleavage, the D macromere has not been specified yet. It will be specified after the formation of the third quartet of micromeres. Unequal cleavage occurs in two ways: asymmetric positioning of the mitotic spindle, or through the formation of a polar lobe (PL).
Specification of the D macromere and is an important aspect of spiralian development. Although the primary axis, animal-vegetal, is determined during oogenesis, the secondary axis, dorsal-ventral, is determined by the specification of the D quadrant.[7] The D macromere facilitates cell divisions that differ from those produced by the other three macromeres. Cells of the D quadrant give rise to dorsal and posterior structures of the spiralian.[7] Two known mechanisms exist to specify the D quadrant. These mechanisms include equal cleavage and unequal cleavage.
In equal cleavage, the first two cell divisions produce four macromeres that are indistinguishable from one another. Each macromere has the potential of becoming the D macromere.[6] After the formation of the third quartet, one of the macromeres initiates maximum contact with the overlying micromeres in the animal pole of the embryo.[6][7] This contact is required to distinguish one macromere as the official D quadrant blastomere. In equally cleaving spiral embryos, the D quadrant is not specified until after the formation of the third quartet, when contact with the micromeres dictates one cell to become the future D blastomere. Once specified, the D blastomere signals to surrounding micromeres to lay out their cell fates.[7]
In unequal cleavage, the first two cell divisions are unequal producing four cells in which one cell is bigger than the other three. This larger cell is specified as the D macromere.[6][7] Unlike equally cleaving spiralians, the D macromere is specified at the four-cell stage during unequal cleavage. Unequal cleavage can occur in two ways. One method involves asymmetric positioning of the cleavage spindle.[7] This occurs when the aster at one pole attaches to the cell membrane, causing it to be much smaller than the aster at the other pole.[6] This results in an unequal cytokinesis, in which both macromeres inherit part of the animal region of the egg, but only the bigger macromere inherits the vegetal region.[6] The second mechanism of unequal cleavage involves the production of an enucleate, membrane bound, cytoplasmic protrusion, called a polar lobe.[6] This polar lobe forms at the vegetal pole during cleavage, and then gets shunted to the D blastomere.[5][6] The polar lobe contains vegetal cytoplasm, which becomes inherited by the future D macromere.[7]
https://en.wikipedia.org/&hellip/File:Equal_vs_unequal_cleavage.j&hellip
D quadrant specification through equal and unequal cleavage mechanisms. At the 4-cell stage of equal cleavage, the D macromere has not been specified yet. It will be specified after the formation of the third quartet of micromeres. Unequal cleavage occurs in two ways: asymmetric positioning of the mitotic spindle, or through the formation of a polar lobe (PL).

D quadrant specification through equal and unequal cleavage mechanisms. At the 4-cell stage of equal cleavage, the D macromere has not been specified yet. It will be specified after the formation of the third quartet of micromeres. Unequal cleavage occurs in two ways: asymmetric positioning of the mitotic spindle, or through the formation of a polar lobe (PL).

In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells (cells with a small even distribution of yolk) or in mesolecithal cells (moderate amount of yolk in a gradient) &ndash bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage.[2] These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.

The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage, the divisions of the blastomeres are complete and separate compared with bilateral meroblastic cleavage, in which the blastomeres stay partially connected.

Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.

Mammals display rotational cleavage, and an isolecithal distribution of yolk (sparsely and evenly distributed). Because the cells have only a small amount of yolk, they require immediate implantation onto the uterine wall in order to receive nutrients.

Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells. The way in which this cleavage differs is that one of the daughter cells divides meridionally, whilst the other divides equatorially.

Spiral cleavage is conserved between many members of the lophotrochozoan taxa, referred to as Spiralia.[3] Most spiralians undergo equal spiral cleavage, although some undergo unequal cleavage (see below).[4] This group includes annelids, molluscs, and sipuncula. Spiral cleavage can vary between species, but generally the first two cell divisions result in four macromeres, also called blastomeres, (A, B, C, D) each representing one quadrant of the embryo. These first two cleavages are oriented in planes that occur at right angles parallel to the animal-vegetal axis of the zygote.[3] At the 4-cell stage, the A and C macromeres meet at the animal pole, creating the animal cross-furrow, while the B and D macromeres meet at the vegetal pole, creating the vegetal cross-furrow.[5] With each successive cleavage cycle, the macromeres give rise to quartets of smaller micromeres at the animal pole.[6][7] The divisions that produce these quartets occur at an oblique angle, an angle that is not a multiple of 90°, to the animal-vegetal axis.[7] Each quartet of micromeres is rotated relative to their parent macromere, and the chirality of this rotation differs between odd and even numbered quartets, meaning that there is alternating symmetry between the odd and even quartets.[3] In other words, the orientation of divisions that produces each quartet alternates between being clockwise and counterclockwise with respect to the animal pole.[7] The alternating cleavage pattern that occurs as the quartets are generated produces quartets of micromeres that reside in the cleavage furrows of the four macromeres.[5] When viewed from the animal pole, this arrangement of cells displays a spiral pattern.

D quadrant specification through equal and unequal cleavage mechanisms. At the 4-cell stage of equal cleavage, the D macromere has not been specified yet. It will be specified after the formation of the third quartet of micromeres. Unequal cleavage occurs in two ways: asymmetric positioning of the mitotic spindle, or through the formation of a polar lobe (PL).

Specification of the D macromere and is an important aspect of spiralian development. Although the primary axis, animal-vegetal, is determined during oogenesis, the secondary axis, dorsal-ventral, is determined by the specification of the D quadrant.[7] The D macromere facilitates cell divisions that differ from those produced by the other three macromeres. Cells of the D quadrant give rise to dorsal and posterior structures of the spiralian.[7] Two known mechanisms exist to specify the D quadrant. These mechanisms include equal cleavage and unequal cleavage.

In equal cleavage, the first two cell divisions produce four macromeres that are indistinguishable from one another. Each macromere has the potential of becoming the D macromere.[6] After the formation of the third quartet, one of the macromeres initiates maximum contact with the overlying micromeres in the animal pole of the embryo.[6][7] This contact is required to distinguish one macromere as the official D quadrant blastomere. In equally cleaving spiral embryos, the D quadrant is not specified until after the formation of the third quartet, when contact with the micromeres dictates one cell to become the future D blastomere. Once specified, the D blastomere signals to surrounding micromeres to lay out their cell fates.[7]

In unequal cleavage, the first two cell divisions are unequal producing four cells in which one cell is bigger than the other three. This larger cell is specified as the D macromere.[6][7] Unlike equally cleaving spiralians, the D macromere is specified at the four-cell stage during unequal cleavage. Unequal cleavage can occur in two ways. One method involves asymmetric positioning of the cleavage spindle.[7] This occurs when the aster at one pole attaches to the cell membrane, causing it to be much smaller than the aster at the other pole.[6] This results in an unequal cytokinesis, in which both macromeres inherit part of the animal region of the egg, but only the bigger macromere inherits the vegetal region.[6] The second mechanism of unequal cleavage involves the production of an enucleate, membrane bound, cytoplasmic protrusion, called a polar lobe.[6] This polar lobe forms at the vegetal pole during cleavage, and then gets shunted to the D blastomere.[5][6] The polar lobe contains vegetal cytoplasm, which becomes inherited by the future D macromere.[7]

CHROMOSOMES HAVE A "FOUR ARM STRUCTURE"- THEY ARE QUADRANTS

Compaction of the duplicated chromosomes during cell division (mitosis or meiosis) results either in a four-arm structure (pictured to the right) if the centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends. Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may unexpectedly evade apoptosis, leading to the progression of cancer.

FOUR CELL DIVISIONS OCCUR RESULTING IN A DENSE BALL 16 CELLS- 16 SQUARES QMR

Cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is called cleavage. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial)[1].

16 CELL- 32 CELL- 64 CELL- 128 CELL- ALL QUADRANT NUMBERS

The simplest of the Volvocaeans are ordered assemblies of cells, each similar to the related unicellar protist Chlamydomonas and embedded in a gelatinous matrix. In the genus Gonium, for example, each individual organism is a flat plate consisting of 4 to 16 separate cells, each with two flagella. Similarly, the genera Eudorina and Pandorina form hollow spheres, the former consisting of 16 cells, the latter of 32 to 64 cells. In these genera each cell can reproduce a new organism by mitosis.[1]

Other genera of Volvocaceans represent another principle of biological development as each organism develops differented cell types. In Pleodorina and Volvox, most cells are somatic and only a few are reproductive. In Pleodorina californica a colony normally has either 128 or 64 cells, of which those in the anterior region have only a somatic function, while those in the posterior region can reproduce the ratio being 3:5. In Volvox only very few cells are able to reproduce new individuals, and in some species of Volvox the reproductive cells are derived from cells looking and behaving like somatic cells. In V. carteri, on the other hand, the division of labor is complete with reproductive cells being set aside during cell division, and they never assume somatic functions or develop functional flagella.[1]

Baculoviridae is a family of viruses. Arthropods, lepidoptera, hymenoptera, diptera, and decapoda serve as natural hosts. There are currently 49 species in this family, divided among 4 genera.[1][2]

CONSISTS OF 16, 32, OR 64 CELLS- ALL QUADRANT NUMBERS

Eudorina is a paraphyletic genus in the volovocine green algae clade.[1] Eudorina colonies consist of 16, 32 or 64 individual cells grouped together. Each individual cell contains flagella which allow the colony to move as a whole when the individual cells beat their flagella together. Description by GM Smith (1920, p 95):[2]

16 32 CELLS QUADRANT NUMBERS- FOR MAMMALS IT IS 16 CELLS IT IS A MORULA- 16 CELLS IS SQUARES OF QUADRANT MODEL- FOR AMPHIBIANS IT IS 128 CELLS WHICH IS A QUADRANT NUMBER

A blastocoel (alt. spelling blastocoele, blastocele) is also termed the blastocyst cavity[6] (or cleavage or segmentation cavity) is the name given to the fluid-filled cavity of the blastula (blastocyst) that results from cleavage of the oocyte (ovum) after fertilization.[7][8] It forms during embryogenesis,[8] as what has been termed a "Third Stage" after the single-celled fertilized oocyte (zygote, ovum[9]) has divided into 16-32 cells,[7] via the process of mitosis.[10] It can be described as the first cell cavity formed as the embryo enlarges,[10] the essential precursor for the differentiated, topologically distinct, gastrula[11]

AFTER FOUR DIVISIONS CONSISTS OF 16 BLASTOMERES CALLED MORULA---- 16 SQUARES QUADRANT MODEL

After fertilization, the conceptus travels down the oviduct towards the uterus while continuing to divide[6] mitotically without actually increasing in size, in a process called cleavage.[7] After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula.[8] Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation.[9] When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the embryonic stage of development.

eggs are shed from host at 16 cell stage- 16 squares qmr

This parasite has a direct life cycle with no intermediate hosts. The life cycle takes about 13&ndash15 days to complete.[1][4] Infected mice will pass faeces containing eggs and egg sizes vary between 70&ndash84 micrometres (µm) in length and 37&ndash53 µm in width.[5] Eggs are shed from the host at the 8&ndash16 cell stage and will hatch in the environment, roughly 24 hours after passing through the host.[6] L1 larvae will emerge from the egg and measure between 300&ndash600 µm in length. Three lip-like structures can be seen around a rudimentary mouth. L1 larvae moult to L2 larvae after 2&ndash3 days, entering bacterial-feeding larval stages present in the environment. The L1 stage cuticle will loosen from either end of the larvae but will remain loosely associated with the L2 larvae, becoming an outer sheath up until infection. After 3 days, the L2 partially moults into ensheathed L3, the infective non-feeding stage. Infective larval stages measure between 480&ndash563 µm long.

The Hominidae (/hɒˈmɪnᵻdiː/), whose members are known as great apes[note 1] or hominids, are a taxonomic family of primates that includes seven extant species in four genera: Pongo, the Bornean and Sumatran orangutan Gorilla, the eastern and western gorilla Pan, the common chimpanzee and the bonobo and Homo, the human (and though not extant, the near-human ancestors and relatives (e.g., the Neanderthal)).[1]

"Little Foot" (Stw 573) is the nickname given to a nearly complete Australopithecus fossil skeleton found in 1994&ndash1998 in the cave system of Sterkfontein, South Africa. [1] The fossils were found in a limestone formation in Sterkfontein. The nickname "little foot" was given to the fossil in 1995. From the structure of the four ankle bones they were able to ascertain that the owner was able to walk upright. The recovery of the bones proved extremely difficult and tedious, because they are completely embedded in concrete-like rock. It is due to this that the recovery and excavation of the site took around 15 years to complete.[1]

The four bones of the ankle had been collected already in 1980 but were undetected between numerous other mammal bones. Only after 1992, on initiative by Phillip Tobias, a large rock was blown up in the cave that contained an unusual accumulation of fossils. The fossils recovered were taken from the cave and scrutinized thoroughly by paleoanthropologist Ronald J. Clarke.[2]

In 1994 while searching through museum boxes labelled 'Cercopithecoids' containing fossil fragments, Ronald J. Clarke identified several that were unmistakably hominin. He spotted four left foot bones (the talus, navicular, medial cuneiform and first metatarsal) that were most likely from the same individual.[3] These fragments came from the Silberberg Grotto, a large cavern within the Sterkfontein cave system. They were described as belonging to the genus Australopithecus, and catalogued as Stw 573.[4]

In 1995, the first description of the four first discovered foot bones was published. The authors explained that this Australopithecus specimen walked upright but was also able to live in trees with the help of grasping movements. This would be possible due to the still opposable big toe.

AVICENNAS BOOK ON MEDICINE WAS CENTERED AROUND THE QUADRANT MODEL- THESIS ONE WAS CENTERED AROUND THE FOUR CAUSES OF ARISTOTLE THESIS TWO WAS THE FOUR ELEMENTS- THESIS THREE WAS THE FOUR TEMPERAMENTS- HE THEN DIVIDES LIFE INTO FOUR STAGES AND RELATES THEM TO THE FOUR TEMPERAMENTS- THEN HE DISCUSSES THE FOUR HUMOURS

Thesis I Definition and Scope of Medicine[edit]

Avicenna begins part one by dividing theoretical medicine and medical practice. He describes what he says are the "four causes" of illness, based on Aristotelian philosophy: The material cause, the efficient cause, the formal cause, and the final cause:[8]:29&ndash31

Material Cause Avicenna says that this cause is the human subject itself, the "members or the breath" or "the humours" indirectly.

Efficient Cause The efficient cause is broken up into two categories: The first is "Extrinsic", or the sources external to the human body such as air or the region we live in. The second efficient cause is the "Intrinsic", or the internal sources such as our sleep and "its opposite-the waking state", the "different periods of life", habits, and race.

Formal Cause The formal cause is what Avicenna called "the constitutions the compositions". According to Oskar Cameron Gruner, who provides a treatise within Avicenna's Canon of Medicine, this was in agreement with Galen who believed that the formal cause of illness is based upon the individual's temperament.

Final Cause The final cause is given as "the actions or functions".

Thesis II The Elements of Cosmology[edit]

Avicenna's thesis on the elements of the cosmos is described by Gruner as "the foundation of the whole Canon".[8]:39 Avicenna insists here that a physician must assume the four elements that are described by natural philosophy,[8]:34 although Avicenna makes it clear that he distinguishes between the "simple" element, not mixed with anything else, and what we actually experience as water or air, such as the sea or the atmosphere. The elements we experience are mixed with small amounts of other elements and are therefore not the pure elemental substances.[8]:202 The "light" elements are fire and air, while the "heavy" are earth and water:

The Earth Avicenna upholds Aristotelian philosophy by describing Earth as an element that is geocentric. The Earth is at rest, and other things tend towards it because of its intrinsic weight. It is cold and dry.[8]:35

The Water Water is described as being exterior to the sphere of the Earth and interior to the sphere of the Air, because of its relative density. It is cold and moist. "Being moist, shapes can be readily fashioned (with it), and as easily lost (and resolved)."[8]:35

The Air The position of Air above Water and beneath Fire is "due to its relative lightness". It is "hot and moist", and its effect is to "rarefy" and make things "softer".[8]:36

The (sphere of the) Fire Fire is higher than the other elements, "for it reaches to the world of the heavens". It is hot and dry it traverses the substance of the air, and subdues the coldness of the two heavy elements "by this power it brings the elementary properties into harmony."[8]:37

B. Compound "intemperaments"

The compound intemperaments are where two things are wrong with the temperament, i.e. hotter and moister hotter and drier colder and moister colder and drier. There are only four because something cannot be simultaneously hotter and colder or drier and moister. The four simple temperaments and four compound intemperaments can each be divided into "Those apart from any material substance" and "Those in which some material substance is concerned", for a total of sixteen intemperaments. Examples of the sixteen intemperaments are provided in the "third and fourth volumes."[8]:64

III The Temperaments Belonging to Age[edit]

The Canon divides life into four "periods" and then subdivides the first period into five separate categories.

The following table is provided for the four periods of life:[8]:68

Period Title Name Year of Age

I The Period of Growth Adolescence Up to 30

II The Prime of Life Period of beauty Up to 35 or 40

III Elderly life Period of decline. Senescence. Up to about 60

IV Decrepit Age Senility To the end of life

The Canon of Medicine is based upon the Four Humours of Hippocratic medicine, but refined in various ways. In disease pathogenesis, for example, Avicenna "added his own view of different types of spirits (or vital life essences) and souls, whose disturbances might lead to bodily diseases because of a close association between them and such master organs as the brain and heart".[11] An element of such belief is apparent in the chapter of al-Lawa", which relates "the manifestations to an interruption of vital life essence to the brain." He combined his own view with that of the Four Humours to establish a new doctrine to explain the mechanisms of various diseases in another work he wrote, Treatise on Pulse:[citation needed]

&ldquoFrom mixture of the four [humors] in different weights, [God the most high] created different organs one with more blood like muscle, one with more black bile like bone, one with more phlegm like brain, and one with more yellow bile like lung.

[God the most high] created the souls from the softness of humors each soul has its own weight and amalgamation. The generation and nourishment of proper soul takes place in the heart it resides in the heart and arteries, and is transmitted from the heart to the organs through the arteries. At first, it [proper soul] enters the master organs such as the brain, liver or reproductive organs from there it goes to other organs while the nature of the soul is being modified in each [of them]. As long as [the soul] is in the heart, it is quite warm, with the nature of fire, and the softness of bile is dominant. Then, that part which goes to the brain to keep it vital and functioning, becomes colder and wetter, and in its composition the serous softness and phlegm vapor dominate. That part, which enters the liver to keep its vitality and functions, becomes softer, warmer and sensibly wet, and in its composition the softness of air and vapor of blood dominate.

In general, there are four types of proper spirit: One is brutal spirit residing in the heart and it is the origin of all spirits. Another &ndash as physicians refer to it &ndash is sensual spirit residing in the brain. The third &ndash as physicians refer to it &ndash is natural spirit residing in the liver. The fourth is generative &ndash i.e. procreative &ndash spirits residing in the gonads. These four spirits go-between the soul of absolute purity and the body of absolute impurity.&rdquo

Definition of body fluid[edit]

The Canon defines a humour as "that fluid, moist 'body' into which our aliment is transformed",[8]:77[12] and lists the four primary types of fluids as sanguineous, serous, bilious, and atrabilious. The secondary fluids are separated into "non-excrementitious" and "excrementitious".

THE FOUR HUMOURS DOMINATED MEDICINE FOR MOST OF HISTORY- IT IS BASED ON QUADRANT DICHOTOMIES

The concept of four humors may have origins in Ancient Egyptian medicine[19] or Mesopotamia,[20] though it was not systemized until ancient Greek thinkers[21] around 400 BC directly linked it with the popular theory of the four elements: earth, fire, water and air (Empedocles).

Fåhræus (1921), a Swedish physician who devised the erythrocyte sedimentation rate, suggested that the four humours were based upon the observation of blood clotting in a transparent container. When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen. A dark clot forms at the bottom (the "black bile"). Above the clot is a layer of red blood cells (the "blood"). Above this is a whitish layer of white blood cells (the "phlegm"). The top layer is clear yellow serum (the "yellow bile").[22]

See also: Medicine in medieval Islam and Unani

Medieval medical tradition in the "Golden Age of Islam" adopted the theory of humorism from Greco-Roman medicine, notably via the Persian polymath Avicenna's The Canon of Medicine (1025). Avicenna summarized the four humors and temperaments as follows:[25]

Avicenna's four humors and temperaments

Evidence Hot Cold Moist Dry

Morbid states Inflammations become febrile Fevers related to serious humor, rheumatism Lassitude Loss of vigour

Functional power Deficient energy Deficient digestive power Difficult digestion

Subjective sensations Bitter taste, excessive thirst, burning at cardia Lack of desire for fluids Mucoid salivation, sleepiness Insomnia, wakefulness

Physical signs High pulse rate, lassitude Flaccid joints Diarrhea, swollen eyelids, rough skin, acquired habit rough skin, acquired habit

Foods and medicines Calefacients harmful, infrigidants[26] beneficial Infrigidants harmful, calefacients beneficial Moist articles harmful Dry regimen harmful, humectants beneficial

Relation to weather Worse in summer Worse in winter Bad in autumn

The four 'humours' or temperaments (Clockwise from top right choleric melancholic sanguine phlegmatic).

The comedy of humours is a genre of dramatic comedy that focuses on a character or range of characters, each of whom exhibits two or more overriding traits or 'humours' that dominates their personality, desires and conduct. This comic technique may be found in Aristophanes, but the English playwrights Ben Jonson and George Chapman popularized the genre in the closing years of the sixteenth century. In the later half of the seventeenth century, it was combined with the comedy of manners in Restoration comedy.

Four temperaments is a theory of psychology about personality. It suggests that four bodily fluids affect human personality traits and behaviour. The temperaments are sanguine, choleric, melancholy, and phlegmatic.

Humours[change | change source]

Galen, a doctor from the Roman empire added characteristics to bodily fluids:

Yellow bile - Warm and Dry

Galen thought that to cure illness you need to rebalance the fluids. If you, for example, have too much blood, you will need a remedy which is the opposite of warm and damp something cold and dry.

Humour Season Element Organ Qualities Ancient name Modern MBTI Ancient characteristics

Blood spring air liver warm & moist sanguine artisan SP courageous, hopeful, amorous

Yellow bile summer fire spleen warm & dry choleric idealist NF easily angered, bad tempered

Black bile autumn earth gall bladder cold & dry melancholic guardian SJ despondent, sleepless, irritable

Phlegm winter water brain/lungs cold & moist phlegmatic rational NT calm, unemotional

CHLOROPHYL AND HEMOGLOBIN BOTH MERKABA STAR TETRAHEDRON- TETRA IS FOUR----
https://metatranspiration.com/&hellip/peace-and-tetrahedron-the-&hellip/
One of the things that has fascinated me over the years is that the fingerprint of the Creator is everywhere. Back in the early &lsquo90s when I was struggling with an overgrowth of systemic yeast, I tried blue-green algae. Algae is a superfood. Unless it is harvested from a clean source and processed properly, I do not recommend its use. One of the things that struck me at the time was that the nucleus of chlorophyll and the nucleus of hemoglobin are almost identical! The only difference is the center atom &ndash in chlorophyll it is magnesium, in hemoglobin it is iron. But the most remarkable thing is that both are Star Tetrahedron. I call the Star Tetrahedron the fingerprint of God.

This hair typing system is the most widely used system to classify hair. The system was created by the hairstylist of Oprah Winfrey, Andre Walker. According to this system there are four types of hair: straight, wavy, curly, kinky.

Type 1 is straight hair, which reflects the most sheen and also the most resilient hair of all of the hair types. It is hard to damage and immensely difficult to curl this hair texture. Because the sebum easily spreads from the scalp to the ends without curls or kinks to interrupt its path, it is the most oily hair texture of all.

Type 2 is wavy hair, whose texture and sheen ranges somewhere between straight and curly hair. Wavy hair is also more likely to become frizzy than straight hair. While type A waves can easily alternate between straight and curly styles, type B and C Wavy hair is resistant to styling.

Type 3 is curly hair known to have an S-shape. The curl pattern may resemble a lowercase "s", uppercase "S", or sometimes an uppercase "Z".[citation needed] This hair type is usually voluminous, "climate dependent (humidity = frizz), and damage prone." Lack of proper care causes less defined curls.

Type 4 is kinky hair, which features a tightly coiled curl pattern (or no discernible curl pattern at all) that is often fragile with a very high density. This type of hair shrinks when wet and because it has fewer cuticle layers than other hair types it is more susceptible to damage.

FOUR TYPES OF HAIR CONDITIONING

Greasy hair is a hair condition which is common in humans, one of four main four types of hair conditioning&mdash normal, greasy, dry and greasy dry

16 SKIN PERSONALITIES- 16 SQUARES QMR

The Baumann Skin Types system is a skin-type classification system defining 16 skin personalities. This classification system was developed in 2004 by University of Miami dermatology professor Leslie Baumann, to subdivide research participants into specific phenotypes. She assigns binary values to four characteristics, so defining sixteen "skin personalities", or "skin types". These have been used in genetic research aimed at identifying the genes that contribute to skin characteristics such as dryness, oiliness, aging, pigmentation and sensitivity. The a survey-based typing system combines these individual skin attributes into 16 personalities that allow researchers to improve their ability to identify various skin phenotypes and use that knowledge for patient selection for clinical research trials and to recommend proper skincare ingredients and products. The classification system has been adopted by estheticians, dermatologists, consumers and retailers to match cosmeceutical ingredients and skin care products to specific skin types.[1] The type assigned is determined by a self-completed questionnaire, marketed as the "Baumann Skin Type Indicator" (BSTI).[2]

The typing system identifies four key skin attributes (dry/oily, sensitive/resistant, pigmented/non-pigmented and wrinkle-prone/tight), represented as D/O, S/R, P/N, and W/T. Combining these results in 16 possible types. For example, one type is ORPW (oily, resistant, pigmented, wrinkle-prone) while another is DSPT (dry, sensitive, pigmented, tight). More than just combinations of skin attributes, each type experiences different dermatologic problems, which indicate preventative measures and treatment options.[3]

There are 4 unique subtypes of sensitive skin, and they are all different, all sensitive subtypes have inflammation in common.

Acne subtype: Develops acne, whiteheads or blackheads

Rosacea subtype: Experiences recurring flushing, redness and a hot sensation

Stinging subtype: Develops stinging or burning of the skin

Allergic subtype: Develops redness, itching and flaking of the skin

MOST COME FROM FOUR FAMILIES- SAME THING IN THE STOMACH MOST BACTERIA FROM FOUR FAMILIES AND FOUR TYPES OF CELLS

The human skin is a rich environment for microbes.[5][6] Around 1000 species of bacteria from 19 bacterial phyla have been found. Most come from only four phyla: Actinobacteria (51.8%), Firmicutes (24.4%), Proteobacteria (16.5%), and Bacteroidetes (6.3%).

THE EPIDERMIS (SKIN) CONTAINS FOUR CELL TYPES

The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells.

FOUR TYPES OF CELLS IN THE EPIDERMIS (outer layer of skin)

This is the top layer of skin made up of epithelial cells. It does not contain blood vessels. Its main functions are protection, absorption of nutrients, and homeostasis. In structure, it consists of a keratinized stratified squamous epithelium comprising four types of cells: keratinocytes, melanocytes, Merkel cells, and Langerhans' cells.

TRANSCENDENT FOURTH DEGREE BURN

Burns that affect only the superficial skin layers are known as superficial or first-degree burns. They appear red without blisters and pain typically lasts around three days.[3][4] When the injury extends into some of the underlying skin layer, it is a partial-thickness or second-degree burn. Blisters are frequently present and they are often very painful. Healing can require up to eight weeks and scarring may occur. In a full-thickness or third-degree burn, the injury extends to all layers of the skin. Often there is no pain and the burn area is stiff. Healing typically does not occur on its own. A fourth-degree burn additionally involves injury to deeper tissues, such as muscle, tendons, or bone.[3] The burn is often black and frequently leads to loss of the burned part.[3][5]

Type[3] Layers involved Appearance Texture Sensation Healing Time Prognosis Example

Superficial (1st-degree) Epidermis[4] Red without blisters[3] Dry Painful[3] 5&ndash10 days[3][16] Heals well[3] Repeated sunburns increase the risk of skin cancer later in life[17] A sunburn is a typical first-degree burn.

Superficial partial thickness (2nd-degree) Extends into superficial (papillary) dermis[3] Redness with clear blister. Blanches with pressure.[3] Moist[3] Very painful[3] less than 2&ndash3 weeks[3][11] Local infection/cellulitis but no scarring typically[11]

Second-degree burn of the thumb

Deep partial thickness (2nd-degree) Extends into deep (reticular) dermis[3] Yellow or white. Less blanching. May be blistering.[3] Fairly dry[11] Pressure and discomfort[11] 3&ndash8 weeks[3] Scarring, contractures (may require excision and skin grafting)[11] Second-degree burn caused by contact with boiling water

Full thickness (3rd-degree) Extends through entire dermis[3] Stiff and white/brown[3] No blanching[11] Leathery[3] Painless[3] Prolonged (months) and incomplete[3] Scarring, contractures, amputation (early excision recommended)[11] Eight day old third-degree burn caused by motorcycle muffler.

4th-degree Extends through entire skin, and into underlying fat, muscle and bone[3] Black charred with eschar Dry Painless Requires excision[3] Amputation, significant functional impairment, and, in some cases, death.[3]

FACE SKIN HAS UP TO 16 LAYERS- 16 SQUARES QMR

The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, which has up to 16 layers. With light skin color, the lip skin contains fewer melanocytes (cells which produce melanin pigment, which give skin its color). Because of this, the blood vessels appear through the skin of the lips, which leads to their notable red coloring. With darker skin color this effect is less prominent, as in this case the skin of the lips contains more melanin and thus is visually darker. The skin of the lip forms the border between the exterior skin of the face, and the interior mucous membrane of the inside of the mouth.

Hair highlighting/lowlighting is changing a person's hair color, using lightener or haircolor to color hair strands. There are four basic types of highlights: foil highlights, hair painting, frosting, and chunking. Highlights can be done in natural or unnatural colors. Color highlights come in four categories: temporary, semi-permanent, demi-permanent and permanent. Hair lightened with bleach or permanent color will be permanent until new growth begins to growth. Highlighted hair will make the hair appear fuller. Therefore, it is recommended on people with thin and fine hair. It also recommended for people with at least 50 % gray for easy blending and to diminish the line of demarcation once the new growth is showing.

"Four Eleven Forty-Four", or "4-11-44" is a phrase that has been used repeatedly in popular music and as a reference to numbers allegedly chosen by poor African Americans for the purpose of gambling on lotteries.

A concept of four unique hues of psychologist Charles Hubbard Judd (1917)

Unique hue is a term used in certain theories of color vision, which implies that human perception distinguishes between "unique" (psychologically primary) and composite (mixed) hues.[1] A unique hue is defined as a color which an observer perceives as a pure, without any admixture of the other colors.[2] There is a great deal of variability when defining unique hues experimentally.[3] Often the results show a great deal of interobserver and intraobserver variability leading to much debate on the number of unique hues.[4] Another source of variability is environmental factors in color naming. Despite the inconsistencies, often four color perceptions are associated as unique &ldquored&rdquo, &ldquogreen&rdquo, &ldquoblue&rdquo, and &ldquoyellow&rdquo.

A concept of four unique hues of psychologist Charles Hubbard Judd (1917)

Chestnuts belong to the family Fagaceae, which also includes oaks and beeches. The four main species are commonly known as European, Chinese, Japanese, and American chestnuts, some species called chinkapin or chinquapin:[4]

Mana means &lsquofour&rsquo in Shona, in reference to the four large permanent pools formed by the meanderings of the middle Zambezi. These 2,500 square kilometres of river frontage, islands, sandbanks and pools, flanked by forests of mahogany, wild figs, ebonies and baobabs, is one of the least developed national parks in Southern Africa. It has the country&rsquos biggest concentration of hippopotami and crocodiles and large dry season mammal populations of the zebra, elephant and Cape buffalo. The area is also home to other threatened species including the lion, cheetah, Cape wild dog, and near-threatened species including leopard and the brown hyena.

FOUR STAGE HOMOSEXUAL IDENTITY

In 1989, Troiden proposed a four-stage model for the development of homosexual sexual identity.[120] The first stage, known as sensitization, usually starts in childhood, and is marked by the child's becoming aware of same-sex attractions. The second stage, identity confusion, tends to occur a few years later. In this stage, the youth is overwhelmed by feelings of inner turmoil regarding their sexual orientation, and begins to engage sexual experiences with same-sex partners. In the third stage of identity assumption, which usually takes place a few years after the adolescent has left home, adolescents begin to come out to their family and close friends, and assumes a self-definition as gay, lesbian, or bisexual.[121] In the final stage, known as commitment, the young adult adopts their sexual identity as a lifestyle. Therefore, this model estimates that the process of coming out begins in childhood, and continues through the early to mid 20s. This model has been contested, and alternate ideas have been explored in recent years.

"Sixteen Military Wives", sometimes referred to as "16 by 32", is a single released by The Decemberists from their third album, Picaresque.

Eventually, the sport evolved and dogs were divided into four groups: those who hunted rabbits, which was not governed by rules those who coursed hare, for which a set of rules was established those trained to the rag and those trained to chase a mechanical lure in a fashion similar to greyhound races.[9] Few of the Whippets of any of the four types were purebred, as maintaining a purebred bloodline was not considered as important as breeding dogs that could win races.[9] Many racing dogs were part terrier, part Greyhound, or part Lurcher.[9

During the Pleistocene era, four species of tapirs are known to have inhabited the North American continent. Along with T. merriami, Tapirus californicus also lived in California, Tapirus veroensis was found in Florida, Georgia, Kansas, Missouri and Tennessee, and Tapirus copei was found from Pennsylvania to Florida.[4]

First discovered and described in 1921 by American vertebrate paleontologist Childs Frick, T. merriami lived at the same time, and perhaps many of the same locations, as T. californicus,[5] but is believed to have preferred more inland habitats of southern California and Arizona. Like T. californicus and all living tapirs, it is believed to have been a relatively solitary species. Of the four known Pleistocene-era tapirs found on the North American continent, T. merriami was the largest.[4] T. merriami was a stout-bodied herbivore with short legs, a large, tapering head, and a short, muscular proboscis adept at stripping leaves from shrubs.[6]

THERE ARE FOUR LEVELS OF NUCLEIC ACID STRUCTURE- THE FOURTH IS TRANSCENDENT- ALSO IT DISCUSSES TETRALOOPS- TETRA IS FOUR

Nucleic acid structure refers to the structure of nucleic acids such as DNA and RNA. Chemically speaking, DNA and RNA are very similar. Nucleic acid structure is often divided into four different levels: primary, secondary, tertiary and quaternary.

The secondary structure of RNA consists of a single polynucleotide. Base pairing in RNA occurs when RNA folds between complementarity regions. Both single- and double-stranded regions are often found in RNA molecules. The antiparallel strands form a helical shape.[3] The four basic elements in the secondary structure of RNA are helices, loops, bulges, and junctions. Stem-loop or hairpin loop is the most common element of RNA secondary structure.[8] Stem-loop is formed when the RNA chains fold back on themselves to form a double helical tract called the stem, the unpaired nucleotides forms single stranded region called the loop.[9] Secondary structure of RNA can be predicted by experimental data on the secondary structure elements, helices, loops and bulges. Bulges and internal loops are formed by separation of the double helical tract on either one strand (bulge) or on both strands (internal loops) by unpaired nucleotides. A tetraloop is a four-base pairs hairpin RNA structure. There are three common families of tetraloop in ribosomal RNA: UNCG, GNRA, and CUUG (N is one of the four nucleotides and R is a purine).UNCG is the most stable tetraloop.[10] Pseudoknot is a RNA secondary structure first identified in turnip yellow mosaic virus.[11] Pseudoknots are formed when nucleotides from the hairpin loop pairs with a single stranded region outside of the hairpin to form a helical segment. H-type fold pseudoknots are best characterized. In H-type fold, nucleotides in the hairpin loop pairs with the bases outside the hairpin stem forming second stem and loop. This causes formation of pseudoknots with two stems and two loops.[12] Pseudoknots are functional elements in RNA structure having diverse function and found in most classes of RNA. DotKnot-PW method is used for comparative pseudoknots prediction. The main points in the DotKnot-PW method is scoring the similarities found in stems, secondary elements and H-type pseudoknots.[13]

FOUR STRAND QUADRANT SHAPE NUCLEIC ACID DESIGN

Nucleic acid design can be used to create nucleic acid complexes with complicated secondary structures such as this four-arm junction. These four strands associate into this structure because it maximizes the number of correct base pairs, with A's matched to T's and C's matched to G's. Image from Mao, 2004.[5]

Condon DE, Kennedy SD, Mort BC, Kierzek R, Yildirim I, Turner DH (June 2015). "Stacking in RNA: NMR of Four Tetramers Benchmark Molecular Dynamics". Journal of Chemical Theory and Computation. 11 (6): 2729&ndash2742. doi:10.1021/ct501025q. PMC 4463549Freely accessible. PMID 26082675.

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QUADRUPLEXES IS TRANSCENDENT- IT IS THE HIGHEST IT GOES- QUAD IS FOUR

Besides double helices and the above-mentioned triplexes, RNA and DNA can both also form quadruple helices. There are diverse structures of RNA base quadruplexes. Four consecutive guanine residues can form a quadruplex in RNA by Hoogsteen hydrogen bonds to form a &ldquoHoogsteen ring&rdquo (See Figure).[11] G-C and A-U pairs can also form base quadruplex with a combination of Watson-Crick pairing and noncanonical pairing in the minor groove.[14]

The core of malachite green aptamer is also a kind of base quadruplex with a different hydrogen bonding pattern (See Figure).[12] The quadruplex can repeat several times consecutively, producing an immensely stable structure.

The unique structure of quadruplex regions in RNA may serve different functions in a biological system. Two important functions are the binding potential with ligands or proteins, and its ability to stabilize the whole tertiary structure of DNA or RNA. The strong structure can inhibit or modulate transcription and replication, such as in the telomeres of chromosomes and the UTR of mRNA.[15] The base identity is important towards ligand binding. The G-quartet typically binds monovalent cations such as potassium, while other bases can bind numerous other ligands such as hypoxanthine in a U-U-C-U quadruplex.[14]

Along with these functions, the G-quadruplex in the mRNA around the ribosome binding regions could serve as a regulator of gene expression in bacteria.[16] There may be more interesting structures and functions yet to be discovered in vivo.

QUADRUPLEX HIGHEST LEVEL- FOURTH TRANSCENDENT

It also allows formation of secondary structures of single stranded DNA and RNA G-rich called G-quadruplexes (G4-DNA and G4-RNA) at least in vitro. It needs four triplets of G, separated by short spacers. This permits assembly of planar quartets which are composed of stacked associations of hoogsteen bonded guanine molecules.[5]

The stability of the loop also influences the formation of the stem-loop structure. "Loops" that are less than three bases long are sterically impossible and do not form. Large loops with no secondary structure of their own (such as pseudoknot pairing) are also unstable. Optimal loop length tends to be about 4-8 bases long. One common loop with the sequence UNCG is known as the "tetraloop" and is particularly stable due to the base-stacking interactions of its component nucleotides.

TETRALOOP INTERACTION TETRA IS FOUR

Stick representation of a GAAA tetraloop - an example from the GNRA tetraloop family.[29]

Tetraloop-receptor interactions combine base-pairing and stacking interactions between the loop nucleotides of a tetraloop motif and a receptor motif located within an RNA duplex, creating a tertiary contact that stabilizes the global tertiary fold of an RNA molecule. Tetraloops are also possible structures in DNA duplexes.[30]

Stem-loops can vary greatly in size and sequence, but tetraloops of four nucleotides are very common and they usually belong to one of three categories, based on sequence.[31] These three families are the CUYG, UNCG and GNRA (see figure on the right) tetraloops.[32] In each of these tetraloop families, the second and third nucleotides form a turn in the RNA strand and a base-pair between the first and fourth nucleotides stabilizes the stemloop structure. It has been determined, in general, that the stability of the tetraloop depends on the composition of bases within the loop and on the composition of this "closing base pair".[33] The GNRA family of tetraloops is the most commonly observed within Tetraloop-receptor interactions.

GAAA Tetraloop and Receptor: Stick representation of tetraloop (yellow) and its receptor, showing both Watson-Crick and Hoogsteen base-pairing.[29]

&ldquoTetraloop receptor motifs&rdquo are long-range tertiary interactions[34] consisting of hydrogen bonding between the bases in the tetraloop to stemloop sequences in distal sections of the secondary RNA structure.[35] In addition to hydrogen bonding, stacking interactions are an important component of these tertiary interactions. For example, in GNRA-tetraloop interactions, the second nucleotide of the tetraloop stacks directly on an A-platform motif (see above) within the receptor.[24] The sequence of the tetraloop and its receptor often covary so that the same type of tertiary contact can be made with different isoforms of the tetraloop and its cognate receptor.[36]

For example, the self-splicing group I intron relies on tetraloop receptor motifs for its structure and function.[24][35] Specifically, the three adenine residues of the canonical GAAA motif stack on top of the receptor helix and form multiple stabilizing hydrogen bonds with the receptor. The first adenine of the GAAA sequence forms a triple base-pair with the receptor AU bases. The second adenine is stabilized by hydrogen bonds with the same uridine, as well as via its 2'-OH with the receptor and via interactions with the guanine of the GAAA tetraloop. The third adenine forms a triple base pair.

TETRALOOP- TETRA IS THE HIGHEST TETRA IS FOUR

GAAA Tetraloop and Receptor: Stick representation of tetraloop (yellow) and its receptor, showing both Watson-Crick and Hoogsteen base-pairing.[29]

For instance, in the P4-P6 domain of the Tetrahymena thermophila group I intron, several ion-binding sites consist of tandem G-U wobble pairs and tandem G-A mismatches, in which divalent cations interact with the Hoogsteen edge of guanosine via O6 and N7.[48][49][50] Another ion-binding motif in the Tetrahymena group I intron is the A-A platform motif, in which consecutive adenosines in the same strand of RNA form a non-canonical pseudobase pair.[51] Unlike the tandem G-U motif, the A-A platform motif binds preferentially to monovalent cations. In many of these motifs, absence of the monovalent or divalent cations results in either greater flexibility or loss of tertiary structure.

This unfortunate lack of scope would eventually be overcome largely because of two major advancements in nucleic acid research: the identification of ribozymes, and the ability to produce them via in vitro transcription. Subsequent to Tom Cech's publication implicating the Tetrahymena group I intron as an autocatalytic ribozyme,[58] and Sidney Altman's report of catalysis by ribonuclease P RNA,[59] several other catalytic RNAs were identified in the late 1980s,[60] including the hammerhead ribozyme. In 1994, McKay et al. published the structure of a 'hammerhead RNA-DNA ribozyme-inhibitor complex' at 2.6 Ångström resolution, in which the autocatalytic activity of the ribozyme was disrupted via binding to a DNA substrate.[61] In addition to the advances being made in global structure determination via crystallography, the early 1990s also saw the implementation of NMR as a powerful technique in RNA structural biology. Investigations such as this enabled a more precise characterization of the base pairing and base stacking interactions which stabilized the global folds of large RNA molecules.

A MINOR MOTIFS FOUR CLASSES

A-minor motifs have been separated into four classes,[8] types 0 to III, based upon the position of the inserting base relative to the two 2&rsquo-OH&rsquos of the Watson-Crick base pair. In type I and II A-minor motifs, N3 of adenine is inserted deeply within the minor groove of the duplex (see figure: A minor interactions - type II interaction), and there is good shape complementarity with the base pair. Unlike types 0 and III, type I and II interactions are specific for adenine due to hydrogen bonding interactions. In the type III interaction, both the O2' and N3 of the inserting base are associated less closely with the minor groove of the duplex. Type 0 and III motifs are weaker and non-specific because they are mediated by interactions with a single 2&rsquo-OH (see figure: A-minor Interactions - type 0 and type III interactions).

QUADRUPLEX FOUR IS THE HIGHEST- FOUR IS TRANSCENDENT- FIFTH IS QUESTIONABLE

In molecular biology, G-quadruplexes (also known as G4 DNA) are secondary structures[1] formed in nucleic acids by sequences that are rich in guanine. These structures are four stranded helical structures and occur naturally in nature. They are normally located near the ends of the chromosomes or the better known as the telomeric regions and in transcriptional regulatory regions of multiple oncogenes.[2] Four guanine bases can associate through Hoogsteen hydrogen bonding to form a square planar structure called a guanine tetrad, and two or more guanine tetrads can stack on top of each other to form a G-quadruplex. The placement and bonding to form G-quadruplexes are not random and serve very unusual functional purposes. The quadruplex structure is further stabilized by the presence of a cation, especially potassium, which sits in a central channel between each pair of tetrads.[3] They can be formed of DNA, RNA, LNA, and PNA, and may be intramolecular, bimolecular, or tetramolecular.[4] Depending on the direction of the strands or parts of a strand that form the tetrads, structures may be described as parallel or antiparallel. G-quadruplex structures can be computationally predicted from DNA or RNA sequence motifs, but their actual structures can be quite varied within and between the motifs, which can number over 100,000 per genome. Their activities in basic genetic processes are an active area of research in telomere, gene regulation, and functional genomics research (Rhodes et al., NAR 2015).[5]

Structure of a G-quadruplex. Left: a G-tetrad. Right: an intramolecular G-quadruplex

3D Structure of the intramolecular human telomeric G-quadruplex in potassium solution (PDB ID 2HY9). The backbone is represented by a tube. The center of this structure contains three layers of G-tetrads. The hydrogen bonds in these layers are represented by blue dashed lines.

3 Non-telomeric quadruplexes

5 Ligands which bind quadruplexes

6 Quadruplex prediction techniques

9.2 Tools to predict G-quadruplex motifs

The length of the nucleic acid sequences involved in tetrad formation determines how the quadruplex folds. Short sequences, consisting of only a single contiguous run of three or more guanine bases, require four individual strands to form a quadruplex. Such a quadruplex is described as tetramolecular, reflecting the requirement of four separate strands. Longer sequences, which contain two contiguous runs of three or more guanine bases, where the guanine regions are separated by one or more bases, only require two such sequences to provide enough guanine bases to form a quadruplex. These structures, formed from two separate G-rich strands, are termed bimolecular quadruplexes. Finally, sequences which contain four distinct runs of guanine bases can form stable quadruplex structures by themselves, and a quadruplex formed entirely from a single strand is called an intramolecular quadruplex.[6]

NEW GROUP III INTRON COMPOSED OF FOUR GROUPS- THE FOURTH IS THE HIGHEST

Since the original discovery, there have been other reports of Group III twintrons and GroupII/III twintrons in Euglena gracilis chloroplast. In 1993 a new type of complex twintron composed of four individual group III introns has been characterized.[2] The external intron was interrupted by an internal intron containing two additional introns. In 1995 scientists discovered the first non-Euglena twintron in cryptomonad alga Pyrenomonas salina.[3] In 2004, several twintrons were discovered in Drosophila.[4]

QUADRANT BASED ON MATERIAL EATEN AND FEEDING STRATEGY - THE TWO DIMENSIONS MAKES FOUR QUADRANTS

Wayne Getz's consumer categories are based on material eaten (plant: green live, brown dead animal: red live, purple dead or particulate: grey) and feeding strategy (gatherer: lighter shades miner: darker shades).[4]

Cross-Shaped Tree Impacts Ind. Hospital

Some are calling it a natural occuring marvel of inspiration.

A tree in the shape of a cross outside of a hospital in Evansville, Ind. is having a big impact.

One employee at Deaconess Hospital recently discovered the cross while peering out a window.

She said seeing the tree cross changed her bad mood immediately.

Now the tree is getting a lot of attention from employees and patients.

The tree was not sculptured, but just grew that way naturally.

The anterior cruciate ligament (ACL) is one of a pair of cruciate ligaments (the other being the posterior cruciate ligament) in the human knee. They are also called cruciform ligaments as they are arranged in a crossed formation. In the quadruped stifle joint (analogous to the knee), based on its anatomical position, it is also referred to as the cranial cruciate ligament.[1] The anterior cruciate ligament is one of the four main ligaments of the knee, and the ACL provides 85% of the restraining force to anterior tibial displacement at 30 degrees and 90 degrees of knee flexion.[2]

THERE ARE FOUR LIGAMENTS IN THE KNEE AND THE C MEANS CRUCIATE WHICH MEANS CRUCIFORM THE CROSS

The anterior cruciate ligament (ACL) is one of a pair of cruciate ligaments (the other being the posterior cruciate ligament) in the human knee. They are also called cruciform ligaments as they are arranged in a crossed formation. In the quadruped stifle joint (analogous to the knee), based on its anatomical position, it is also referred to as the cranial cruciate ligament.[1] The anterior cruciate ligament is one of the four main ligaments of the knee, and the ACL provides 85% of the restraining force to anterior tibial displacement at 30 degrees and 90 degrees of knee flexion.[2]

CRUCIATE MEANS CRUCIFORM CROSS

Anterior cruciate ligament injury occurs when the biomechanical limits of the ligament are exceeded (over-stretched). The anterior cruciate ligament (ACL) is an important internal stabilizer of the knee joint. The primary function of the ACL is to prevent hyperextension its secondary function is to restrain tibial rotation and varus/valgus stress.[3]

IT IS CALLED CRUCIATE BECAUSE IT HAS FOUR COMPONENTS RESEMBLING A CROSS

The cruciate anastomosis is a circulatory anastomosis in the upper thigh of the inferior gluteal artery, the lateral and medial circumflex femoral arteries, and the first perforating artery of the profunda femoris artery. Also, the anastomotic branch of the posterior branch of the obturator artery.[1] The cruciate anastomosis is clinically relevant because if there is a blockage between the femoral artery and external iliac artery, blood can reach the popliteal artery by means of the anastomosis. The route of blood is through the internal iliac, to the inferior gluteal artery, to a perforating branch of the deep femoral artery, to the lateral circumflex femoral artery, then to its descending branch into the superior lateral genicular artery and thus into the popliteal artery.

The cruciate anastomosis is so-called because it resembles a cross. Its four components are:

lateral circumflex femoral artery (transverse branch of)

medial circumflex femoral artery (transverse branch of)

first perforating artery from profunda femoris (ascending branch of)

The joint is stabilized by paired collateral ligaments which act to prevent abduction/adduction at the joint, as well as paired cruciate ligaments. The cranial cruciate ligament and the caudal cruciate ligament restrict cranial and caudal translation (respectively) of the tibia on the femur. The cranial cruciate also resists over-extension and inward rotation, and is the most commonly damaged stifle ligament in dogs.

CRUCIFORM CARD
https://en.wikipedia.org/wiki/File:Cruciform_Triage_card.jpg
https://en.wikipedia.org/wiki/Triage_tag#Examples
The Cruciform triage card, used in the UK including the North Sea oil industry, by the Royal London Hospital during the 7 July 2005 London bombings and by medical and paramedical organisations worldwide. The International Cruciform has been produced for the Canada/North America (with appropriate terminology adjustments), and translated versions of the card are available for European and Asian markets. The Cruciform Evacuation System is a variation of the system applying Triage to mass planned and unplanned evacuation scenarios (e.g. hospital evacuations).
The Smart Tag from TSG Associates. Adopted by the State of New York in 2004, the British Military in 2002, used by London Ambulance Service in the 7 July 2005 London bombings and by the combined forces Afghanistan in 2006. The tag is also used in New York, Philadelphia, Boston and Nevada, and is mandated for use across the States of Connecticut and Massachusetts.

FOUR SETS OF INVERTED REPEATS

Inverted repeats are a key component of pseudoknots as can be seen in the illustration of a naturally occurring pseudoknot found in the human telomerase RNA component.[17] Four different sets of inverted repeats are involved in this structure. Sets 1 and 2 are the stem of stem-loop A and are part of the loop for stem-loop B. Similarly, sets 3 and 4 are the stem for stem-loop B and are part of the loop for stem-loop A.

Pseudoknot with four sets of inverted repeats. Inverted repeats 1 and 2 create the stem for stem-loop A and are part of the loop for stem-loop B. Similarly, inverted repeats 3 and 4 form the stem for stem-loop B and are part of the loop for stem-loop A.

CRUCIFORM STRUCTURE- TETRAPLEX- TETRAPLEX IS THE HIGHEST TETRA FOUR IS TRANSCENDENT
https://en.wikipedia.org/wiki/File:DNA_palindrome.svg
https://en.wikipedia.org/wiki/Inverted_repeat
The illustration shows an inverted repeat undergoing cruciform extrusion. DNA in the region of the inverted repeat unwinds and then recombines, forming a four-way junction with two stem-loop structures. The cruciform structure occurs because the inverted repeat sequences self-pair to each other on their own strand.[20]

The instability results from the tendency of inverted repeats to fold into hairpin- or cruciform-like DNA structures. These special structures can hinder or confuse DNA replication and other genomic activities.[6] Thus, inverted repeats lead to special configurations in both RNA and DNA that can ultimately cause mutations and disease.[8]

To a large extent, portions of nucleotide repeats are quite often observed as part of rare DNA combinations.[13] The three main repeats which are largely found in particular DNA constructs include the closely precise homopurine-homopyrimidine inverted repeats, which is otherwise referred to as H palindromes, a common occurrence in triple helical H conformations that may comprise either the TAT or CGC nucleotide triads. The others could be described as long inverted repeats having the tendency to produce hairpins and cruciform, and finally direct tandem repeats, which commonly exist in structures described as slipped-loop, cruciform and left-handed Z-DNA.[13]

Extruded cruciforms can lead to frameshift mutations when a DNA sequence has inverted repeats in the form of a palindrome combined with regions of direct repeats on either side. During transcription, slippage and partial dissociation of the polymerase from the template strand can lead to both deletion and insertion mutations.[8

non-B DB A Database for Integrated Annotations and Analysis of non-B DNA Forming Motifs.[21] This database is provided by The Advanced Biomedical Computing Center (ABCC) at then Frederick National Laboratory for Cancer Research (FNLCR). It covers the A-DNA and Z-DNA conformations otherwise known as "non-B DNAs" because they are not the more common B-DNA form of a right-handed Watson-Crick double-helix. These "non-B DNAs" include left-handed Z-DNA, cruciform, triplex, tetraplex and hairpin structures.[21] Searches can be performed on a variety of "repeat types" (including inverted repeats) and on several species.

Pearson, CE Zorbas, H Price, GB Zannis-Hadjopoulos, M (October 1996). "Inverted repeats, stem-loops, and cruciforms: significance for initiation of DNA replication". Journal of cellular biochemistry. 63 (1): 1&ndash22. doi:10.1002/(SICI)1097-4644(199610)63:1<1::AID-JCB1>3.0.CO2-3. PMID 8891900.
^ Jump up to: a b c


Coeloblastula

Lors de l'embryogenèse animale, au cours de la gastrulation, et au moment où s'invagine le pôle végétatif de la cœloblastula, il se produit une ouverture : c'est le blastopore. Celui-ci donne accès à la cavité archentérique située dans le sac invaginé, ébauche du tube digestif Owenia collaris embryos undergo holoblastic spiral cleavage to form a coeloblastula, which, although exceptionally hollow (again like some nemerteans), conforms to the ancestral pattern for annelids (Okada, 1970). Unusual development of the mitraria larva in the polychaete Owenia collari Définition coeloblastule dans le dictionnaire de définitions Reverso, synonymes, voir aussi 'coeloblastula',coelomate',coelomatique', expressions, conjugaison, exemple The coeloblastula is characteristic of certain Coelenterata, lower Arthropoda, Echinodermata, Tunicata, Acrania, Cyclostomata, Acipenseridae, and most Amphibia. It is formed as a result of complete radial cleavage (even or uneven). It resembles a bubble filled with fluid À partir du stade 8-16 blastomères, les cellules se divisent perpendiculairement à la surface de l'embryon et forment une coeloblastula. From the stage of 8-16 blastomeres, the cells divide perpendicularly to the surface of embryos and form a coeloblastula

Coeloblastula definition is - a hollow blastula. Love words? You must — there are over 200,000 words in our free online dictionary, but you are looking for one that's only in the Merriam-Webster Unabridged Dictionary.. Start your free trial today and get unlimited access to America's largest dictionary, with: . More than 250,000 words that aren't in our free dictionar La stomatoblastula est située dans la mésoglée de l' éponge -mère, et que sa sortie vers l'extérieur est accompagnée de cette inversion de membranes des coeloblastula (avec un blastocèle) Cette coeloblastula est déjà présente chez les animaux archaïques diploblastiques (à deux feuillets) : la blastula, composée d'une seule couche de cellules, en général cilliées, entoure le blastocèle

La coeloblastula (coeloblastule) ou archiblastula (archiblastule), également appelée germe de la vessie, est souvent sphérique ou ovoïde et se caractérise par une cavité plus ou moins grande, située au centre ou à l'excentrique, le blastocèle. Elle est souvent considérée comme la blastula typique. Il peut être à une seule couche (par exemple avec de nombreux hydrozoaires, ainsi qu. Le Dictionnaire comporte plus de 122 000 entrées.Il reconnaît les formes fléchies (féminin, pluriel, conjugaison des verbes).Les noms propres ne sont pas pris en compte. Pour accéder au Dictionnaire, il vous suffit de cliquer sur le bouton Dictionnaire dans la barre du menu de recherche.Tapez votre recherche dans la boîte de saisie prévue à cet effet et cliquez sur le bouton RECHERCHER Définition coeloblastula. avec . coeloblastula est employé comme nom féminin singulier. Employé comme nom. 1. en embryologie, résultat de segmentation dans l'œuf, produisant le blastopore. Quelques mots au hasard. polyédrique - agrométéorologique - paucis - mongolienne - albicore - italo-tunisien - orageusement - aguetter - masques-heaumes - limerick - aide-poseur - chinchilla. La segmentation (ou clivage) correspond aux premières mitoses du zygote (oeuf fécondé) en blastomères, sans augmentation du volume de départ de l'oeuf The calcareous sponge Leucosolenia laxa releases free-swimming hollow larvae called coeloblastulae that are the characteristic larvae of the subclass Calcinea. Although the coeloblastula is a major type of sponge larva, our knowledge about its development is scanty. Detailed electron microscopic studies on the metamorphosis of the coeloblastula revealed that the larva consists of four types of.

Définition de coeloblastula - Encyclopædia Universali

  • coeloblastula (plural coeloblastulas or coeloblastulae) A hollow blastula formed from blastomeres. 2008, Tracey Irene Smart, Reproductive and Larval Biology of the Northeastern Pacific Polychaete Owenia collaris (Family Oweniidae) in Coos Bay, OR (page 8
  • donc coeloblastula n'est qu'une autre appellation de bmastocèle 14/10/2011, 23h52 #3 fattten. Re : Coeloblastule? rectification : coeloblastule = une blastula pourvue de cavité( chez les amphibiens et echinoderme) il existe aussi une blastula depourvu de cavité dite : sterroblastula ( chez les annélide) à partir du stade 16 blastoméres ( puisque la segmentation se fait d'une façon.
  • coeloblastula definition in French dictionary, coeloblastula meaning, synonyms, see also 'cœloblastula',coeloblastule',coelo'. Enrich your vocabulary with the French Definition dictionar
  • Traductions en contexte de coeloblastula through en anglais-français avec Reverso Context : The egg is 267 ± 12 μm in diameter and negatively buoyant with an orangish pink yolky cap at the animal pole which, after fertilization, develops into a coeloblastula through equal, holoblastic cleavages
  • cœloblastula definition, meaning, French dictionary, synonym, see also 'coeloblastula',cœloblastule',cœlo', Reverso dictionary, French definition, French vocabular
  • coe·lom also ce·lom (sē′ləm) n. pl. coeloms or coe·lo·ma·ta (-lə-mä′tə, -măt′ə) also ce·loms or ce·loma·ta The fluid-filled cavity within the body of most multicellular animals, except some invertebrates such as flatworms and cnidarians, that lies between the body wall and the digestive tract and is formed by the splitting of the.

Les oeufs ont 267 ± 12 μm de diamètre et ont une flottabilité négative ils sont munis, au pôle animal, d'un bouchon de vitellus rose-orangé qui, après la fécondation, se développe en une coeloblastula par des clivages holoblastiques égaux. Giga-fren Giga-fre Coeloblastula est la prochaine étape du développement des œufs qui subissent ces cleavaging radiale. Dans les œufs holoblastique, le premier clivage se produit toujours le long de l'axe végétal et l' animal de l'œuf, le deuxième clivage est perpendiculaire à la première. A partir de là, la disposition spatiale des blastomères peut suivre différents modèles, en raison de. Les larves sont d'ailleurs de trois type: Parenchymula, c'est à dire qu'elle est remplie de cellules, Coeloblastula, (coel=creux) c'est à dire qu'elle est creuse à l'intérieur et Amphiblastula c'est à dire qu'elle forme une poche ouverte par un orifice seulement 31/01/2019 umr cnrs 5023 divi biologie des organismes animaux nathalie mondy et pierre marmonier plan du cours passage la pluricellularité diploblastiques e Vivipares, larve typique : coeloblastula ou amphiblastula. Sous-classe: Calcaronea / Calcaronia: Calcaronea / Calcaronia: Spicules à quatre rayons. Le noyau des choanocytes* a une position apicale. Larve de type amphiblastula. Ordre: Leucosolenida: Leucosolénides: Famille: Sycettidae: Sycettidés: Genre: Sycon: Espèce: ciliatum: ANIMAUX . Eponges ou Spongiaires. Éponges calcaires.

Learn how to say/pronounce coeloblastula in American English. Subscribe for more videos Chapitre IV Stade de la Gastrulation. 1. Définition de la gastrulation : La gastrulation est l'ensemble des mouvements cellulaires qui se produisent pour 2 buts Entries with coeloblastula blastula: filled with fluid a blastosphere.Related words & phrases amphiblastula blastulation coeloblastula discoblastula stereoblastula Translations blastula - an early form in. blastocoele: (pl. blastocoeles) The fluid-filled cavity in a blastula Related words & phrases coeloblastula Translations blastocoele - the fluid-filled cavity in a blastul Que vuet dire Blastula Blastula est un mot d'origine Anglaise blastula en Espagnol sa veut dire blástula blastula en Portugais sa veut dire blástul Les oeufs ont 267 ± 12 μm de diamètre et ont une flottabilité négative ils sont munis, au pôle animal, d'un bouchon de vitellus rose-orangé qui, après la fécondation, se développe en une coeloblastula par des clivages holoblastiques égaux

coeloblastulae definition: Noun 1. plural form of coeloblastula.. The coeloblastula is characteristic of certain Coelenterata, lower Arthropoda, Echinodermata, Tunicata, Acrania, Cyclostomata, Acipenseridae, and most Amphibia. It is formed as a result of complete radial cleavage (even or uneven). 4.0 1 vote 1 vote Rate! Rate! Thanks 1. Comments Report Log in to add a comment Srushti01 Expert Hey mate. Here is your answer. A stage of embroic development.

The blastula (from Greek βλαστός (blastos), meaning sprout) is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoel formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells. A coeloblastula is that type of blastula which has a fluid-filled cavity. Since the blastocyst of mammals has a fluid-filled cavity too, can it be called a coeloblastula? My book however mentions the two as different, and it gives the frog as an example of animals having the coeloblastula and mammals as an example of having the blastocyst Coelogastrula definition is - a typical gastrula derived from a coeloblastula. Love words? You must — there are over 200,000 words in our free online dictionary, but you are looking for one that's only in the Merriam-Webster Unabridged Dictionary.. Start your free trial today and get unlimited access to America's largest dictionary, with: . More than 250,000 words that aren't in our free.

Coeloblastula definition of coeloblastula by Medical

  1. ation. Invagination involves the blastula folding in on itself, creating a pocket with an opening. These are known as the archenteron and blastopore, and will become parts of the gut. The in-fold becomes the endoderm, while the outer later becomes the.
  2. Coeloblastula et variantes: B) Ingression de certaines cellules de la paroi pour remplir la cavité Ectoderme = tissu extérieur Endoderme = tissu intérieur remplissant la gastrula Stéréogastrula Etape du développement où les mouvements morphogénétiques sont entamés Cas de nombreux Cnidaires. 13 Développement Les grands plans de développement des euMétazoaires 4) Gastrulation.
  3. Définitions de archiblastula, synonymes, antonymes, dérivés de archiblastula, dictionnaire analogique de archiblastula (anglais

Définition coeloblastule Dictionnaire français Revers

une larve coeloblastula typique (Fig. 1, III) sphérique, ciliée, nageuse et comportant une seule couche de cellules. L'épithélium de la blastula possède des cellules très hautes où le noyau délimite deux zones cyto- plasmiques distinctes : une zone externe étroite, très finement granu­ leuse et très phloxinophile et une zone interne bourrée d'inclusions vitellines de grosse. Embryologie Descriptive. de la fécondation à l'organogénèse - I - Définitions - Généralités. Embryologie : étude du développement de l'embryon (œuf → éclosion/parturion) Blastulation Asst Prof: Chetana Kanekar Ashoka College Of Educatio coeloblastula, metamorphosing larva, and juvenile sponge of Leucosolenia laxa in clear electron micrographs. After settlement, the larval flagellated cells dedifferentiate into a simple cell mass on the substratum thereafter they differ-entiate again into the three principal cell types of a juvenile sponge. We discuss the multipotency of the larval flagel- lated cells of L. laxa by comparing.

2. coeloblastula 3. discoidblastula 4. periblastula 5. blastocyst the blastula of frog with its structure and the fate map of blastula find us on facebook page https://www.facebook.com. a stage in the embryonic development of most arthropods having centrolecithal eggs. The periblastula is a vesicle whose wall consists of one layer of cells and whose cavity is filled with unbroken yolk Disclaimer. All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only Blastula je rané embryonální stadium, které má kulovitý tvar a uvnitř obsahuje dutinu, blastocoel. V klasickém schématu embryonálního vývoje vzniká v procesu blastulace (blastogeneze) z moruly, což je plná koule, a vyvíjí se z ní gastrula.Blastula je typická tím, že v ní začíná docházet k první regulaci buněčného dělení a také začíná vlastní transkripce.

Video: Coeloblastula Article about Coeloblastula by The Free

Coeloblastula - Traduction en anglais - exemples français

  • iature » où tous les organes sont déjà présents
  • m/s n°12, vol.13, décembre 97 1503 L'origine de la gastrulation Herman Denis C 'est en étudiant le dévelop-pement du poulet que les premiers embryologiste
  • 1-Définition la segmentation consiste en une série de divisions morcelant l'œuf en cellules de plus en plus petites appelées blastomères, ce qui aboutit à la formation d'un blastocyste
  • coeloblastula. 67%. C holoblastula. 11%. D stereoblastula. 0%. Solution: Eggs of frog are mesolecithal i.e., they have a moderate amount of yolk. These eggs devide by holoblastic equal division and form a blastula. Blastula contains a cavity called blastocoel which is covered by numerous cells. This type of blastula is known as coeloblastula. You must select option to get answer and solution.

La coeloblastula possède aussi un feuillet externe appelé exoderme. - L'épibolie: Elle touche les stéréoblastrula. Les micro mères qui étaient au pôle animal, prolifèrent en se divisant. Elles glissent jusqu'à recouvrir les macromères, ce qui recouvre l'ensemble des macro mères pour former un archanteron au sein de la blastula. Elles se creusent un intestin primitif. ==> Voir. The coeloblastula is at first spherical, but later flattens in the region of the mesodermal crescent this flattening extends forward on the vegetative side to the chorda‐neural crescent, where the invagination is sharpest. The blastopore is at first triangular in outline, the dorsal lip being formed by the chorda‐neural crescent and the lateral lips by the mesodermal crescent. Later the. une cavité appelée blastocœle au centre de l'embryon : on parle de coeloblastula. Chez les Oiseaux, le mode de segmentation amène à la formation d'annexes embryonnaire discoïdes donc on parle de discoblastula. Chez la drosophile, les noyaux migrent à la périphérie puis le processus de cellularisation mène à la formation d'une périblastula. Quand la segmentation est totale, tout le. La coeloblastula possède aussi un feuillet externe appelé exoderme. - L'épibolie: Elle touche les stéréoblastrula. Les micromères qui étaient au pôle animal, prolifèrent en se divisant. Elles glissent jusqu'à recouvrir les macromères, ce qui recouvre l'ensemble des macromères pour former un archanteron au sein de la blastula. Elles se creusent un intestin primitif. ==> Voir. L'embryogenèse d'Halisarca dujardini de la mer Blanche (Russie) a été étudiée en microscopie photonique et électronique. La segmentation est égale et asynchrone. À partir du stade 8-16 blastomères, les cellules se divisent perpendiculairement à la surface de l'embryon et forment une coeloblastula. Les rares cellules internes de la prélarve proviennent d'une.

Although the coeloblastula is a major type of sponge larva, our knowledge about its development is scanty. Detailed electron microscopic studies on the metamorphosis of the coeloblastula revealed that the larva consists of four types of cells: flagellated cells, bottle cells, vesicular cells, and free cells in a central cavity. The flagellated cells, the principal cell type of the larva, are. Stades Xénope Poisson zèbre Poulet Souris Homme 2 cellules 1,5 h 45 min 5 h 16 / 18 h 30 h début blastula 4,5 h > 2 h 15 et < 3 h 23 / 25 h 3,5 jours 5 jour

L'embryologie est une discipline scientifique qui englobe la description morphologique des transformations de l'œuf fécondé en organisme (embryologie morphologique) et l'étude de leur déterminisme (embryologie causale).). « L'embryologie causale » est plus couramment désignée, depuis les années 1990 et l'avènement de la génétique moléculaire, par le terme de « biologie du. Metamorphosis of Coeloblastula Performed by Multipotential Larval Flagellated Cells in the Calcareous Sponge Leucosolenia laxa. Shigetoyo Amano and Isao Hori. 200(1), pp. 20-32. Abstract | Full Text | PDF (1351 KB) | Permissions -+ Show Abstract. Ecology and Evolution. Interspecific Relationships Between Egg Size and the Level of Parental Investment per Offspring in Echinoderms. L. R. Poriferans (sponges)• 'pore bearing'• Spongocoel• Choanocytes or Collar cells• Archeocytes• Epidermal cells• Has Mesophyl sandwiched between two t

. 1). This larva, which is about 90 [micro]m in length and about 50 [micro in width, is entirely ciliated--including its posterior pole. The coeloblastula contains three additional types of cells: bottle cells, vacuolar cells, and free cells in the central cavity. Figure 1 also. En se divisant, il devient une coeloblastula, sphère ou sphéroïde creux comportant une seule couche de blastomères le plus souvent ciliés, entourant une cavité, dénommée blastoco èle. (œufs alécithes, olig o lécithes, hétérolécithes) - Sterroblastula : Si l'œuf est riche en vitellus. Le blastocoele est obstrué par quelques gros blastomères chargés de réserves. La blastula. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms. Bilateral The.

Coeloblastula Definition of Coeloblastula by Merriam-Webste

  1. An amphiblastula or coeloblastula is produced by calcareous sponges a parenchymella is the larva of the majority of demosponges, but several other larval types are known and a trichimella is exclusive to hexactinellid sponges
  2. Coeloblastula larva 6. Metamorphosis. Date: 20 October 2016: Source: Own work based on: Ereskovsky, A. V. The Comparative Embryology of Sponges. Author: Maxinvestigator: Licensing . I, the copyright holder of this work, hereby publish it under the following license: This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. You are free: to share.
  3. (embryology) An early form in the development of an embryo, consisting of a spherical layer of cells filled with fluid a blastosphere.··(embryology) blastul
  4. Retrouvez la définition du mot peinture-émail dans notre dictionnaire en ligne par la-conjugaion.fr
  5. larvae (coeloblastula, calciblastula, cinctoblastula, and amphiblastula)ortwo-layeredwithoutcavity(parenchymella, hoplitomella, and trichimella). From the larval stage, the attached adult stage is reached by a metamorphosis stage. This is a short stage of postem-bryonic development during which the larva undergoes radical morphological and physiological changes. Some structures, usually larval.

Coeloblastula is a related term of blastula. Noun (wikipedia blastula) (en-noun) (embryology) An early form in the development of an embryo, consisting of a spherical layer of cells filled with fluid a blastosphere .1) that propagates a spatially restricted signal within the extracellular perivitelline space of the syncitial blastoderm embryo [3,4].Co-expression studies in cell culture demonstrated that Gastrulation defective cleaves and activates the Snake zymogen

coeloblastula spelling and pronunciation. Springkleen. Follow. 4 years ago | 21 views. Learn how to spell and pronounce coeloblastula. Report. Browse more videos. Playing next. 12:44. Nintendo vs Playstation vs Xbox: History of GAMING WARS - Primer for the future - GAMING WARS 8. . Semi-thin. Scale bar, 175 pm. Fig. 4. Wrinkled coeloblastulae (W) of Oscarella tuberculata. Semi-thin. Scale bar, 67 pm. Fig. 5.

Coeloblastula Régulière. Œuf Hétérolécithe. Coeloblastula Irrégulière . Elle peut être Radiale ou Spirale ou Rotationnelle. (Voir schéma) 2/ La segmentation partielle : appelé aussi segmentation Méroblastique : c'est uniquement une partie de la cellule qui se divise, cette partie est appelée partie cellulaire. Type de l'œuf . Type de segmentation. Résultat. Œuf. dict.cc English-German Dictionary: Translation for coeloblastula. English-German online dictionary developed to help you share your knowledge with others Coeloblastula: 有腔胞胚 [ゆうこうほうはい] Translations: 1 - 1 / 1. Your Recent Searches . EUdict (European dictionary) is a collection of online dictionaries for the languages spoken mostly in Europe. These dictionaries are the result of the work of many authors who worked very hard and finally offered their product free of charge on the internet thus making it easier to all of. . Here it is shown by using Nomarski optics that development of the freshwater nematode Tobrilus diversipapillatus differs from this pattern in.

Amphiblastula — Wikipédi

  • The fertilized frog egg develops from about 4.5 hours into a coeloblastula with eccentric blastocoel: When segmentation of the egg occurs, dividing it into smaller cellular units, in 8 -cell stage follows the appearance of an internal cavity known as the segmentation cavity or blastocoel
  • Embryogénèse (Ontogénèse (Ensemble (Étapes (Développement post: Embryogénèse (Ontogénèse
  • Coeloblastula • It is hollow sphere and blastocoel is filled with mucopolysaccrides and blastoderm is of single layer of cells. • E.g. Echinoderms 11. Stereoblastula • It is a Solid blastula because there is no blastocoel cavity. • This type is formed due to spiral cleavage. • E.g. Molluscs. 12. Superficial Blastula • It is also called periblastula. • Periblastula is formed in.
  • tradução animal pole em frances, dicionário Ingles - Frances, definição, consulte também 'animal experiment',animal fat',animal liberationist',animal lover

Embryologie générale : segmentation (blastomères, morula

  1. jamais de stade coeloblastula typique. La gastrula didermique se différencie en une larve nageuse, la planula qui, après quelques jours de vie libre, va se fixer au support pour donner naissance à une nouvelle colonie. INTRODUCTION. Les œufs d'Hydractinia se forment au sein d'un gonophore du type hétéromédusoïde. Ayant atteint leur maturité, ils sont pondus, fécondés et se.
  2. BIOUnnes_Blastulasi 1. KELOMPOK 6 FATHURRAHMAN SIDIQ (4411411049) KAMILATUSSANIAH (4411411038) RAFITA FARANTIKA (4411411035) EKA PUTRI S (4411411043) BLASTULASI Fathur Kamila Rafita Eka Putri Biology Non Education Second Group Semarang State Universit
  3. Coeloblastula (f Convergence (f veg Il (f Nemertina (V veliger Crustacea (i Echinodermata (Y Ecdysozoa 0 Ecdysozoa (Y Lophotrocozoa (Y , Lophofrocozoa (f periblastula (Y discoblastula (Y Coeloblastula (i Heterochrony (T vegl (r Divergence (Y an Il (Y (nurse cells) Allometry O . 287F Involution (f Invagination (T Involution -.+S.Ž (Y Invagination 0 Interstitial -Syncytial blastoderm -cellular.
  4. cellulaire, on parle de coeloblastula (ex chez les amphibiens et les échinodermes). Le blastocèle est délimitée par une couche de cellules de type épithélial. Si le blastula est quasiment dépourvu de cavité on parle de sterroblastula (ex chez les annélides). Autres types de blastula (obtenues suite à des segmentations memblastiques)
  5. Blastula, hollow sphere of cells, or blastomeres, produced during the development of an embryo by repeated cleavage of a fertilized egg. The cells of the blastula form an epithelial (covering) layer, called the blastoderm, enclosing a fluid-filled cavity, the blastocoel. After the blastul

Blastula: définition et explication

  1. Coeloblastula Forme larvaire associée au développement ovipare et caractérisée from BIO 2535 at University of Ottaw
  2. Coeloblastula embryos with dividing cell inside of the follicle. a TEM of mitotic coeloblastula ciliated cell with the chromosomes (ch), phagosomes (ph), lipid droplets (l), fc follicular Guancha.
  3. Synonyms for coelom in Free Thesaurus. Antonyms for coelom. 2 synonyms for coelom: celom, celoma. What are synonyms for coelom
  4. iature, et qui grandit.
  5. stereoblastula definition: Noun (plural stereoblastulas or stereoblastulae) 1. A blastula without a clear central cavity.Origin stereo- +‎ blastula..
  6. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms. Bilateral The.
  7. Embrio yang memiliki rongga itu disebut blastula, rongganya disebut blastocoels. Proses pembentukan blastula disebut blastulasi.Blastula merupakan bentuk lanjutan dari morula yang terus mengalami pembelahan

Embryonic development of the northeastern Pacific feather star Florometra serratissima takes place within a ridged fertilization membrane. Cleavage is radial, resulting in a coeloblastula, and gastrulations is by invagination. Cilia are swollen terminally during ciliogenesis whereas fully grown cilia possess several swellings along the length of their shafts . Ocean acidification, or the lowering of seawater pH, is caused by sequestration of atmospheric CO2 into the oceans. This study investigated the effects of present-day pH 8.0, predicted ocean surface pH for the years 2100 and 2300 (pH 7.7 and pH 7.3, respectively) and an extreme pH (pH 7.0) on fertilisation and embryogenesis in the Antarctic nemertean worm Parborlasia corrugatus and sea urchin. Development includes coeloblastula or amphiblastula larva Ex: Clathrina, Leucosolenia, Scypha. Class II: Hexactanellida (Gr. Hex=six Actin=ray) This class includes glass sponges. The sponges of this class are of moderate size. They all are exclusively marine forms living in deep waters. Their skeleton is made up of six-rayed siliceous spicules

Définition de sterroblastula - Encyclopædia Universali

Les oeufs de priapulides sont sphériques petits ( 0.06-0.08 ) , pondus et fécondés dans l'eau de mer subissent une segmentation totale égale et bilatérale , radiaire ( et non spirale) qui aboutit à la formation d'une coeloblastula. La larve, très originale ressemble à l'adulte ( introvert et tronc) , cependant elle est dépourvue de panache caudal et porte une cuirasse formée de deux. » coelo, coeloblastula, coeloblastulae, coeloblastulas, coeloconic, coelodont, coelodonts, coelom, coeloma, coelomata, coelomate. Search. Info. WordSense.eu - English dictionary containing information about the meaning, the spelling and more.We answer the question: What does coelo‎ mean? References . The references include Cambridge Dictionary Online, Centre National de Ressources.

Définition coeloblastula - La conjugaiso

  • adult sponge Amano and Hori amoebocytes amoeboid amphiblastula Anakina anterior-posterior apical aquiferous system archaeocytes asexual reproduction axis basal Bergquist blastomeres blastula Borojevic 1969 Boury-Esnault budding Calcarea Calcaronea Calcinea cavity choanocyte chambers choanocytes choanoderm ciliated ciliated cells cinctoblastula cleavage coeloblastula collagen collencytes.
  • the coeloblastula stage in 5 species of Oscarella (De-mospongiae, Homoscleromorpha). The most distinc-tive feature of early development in homosclero-morphs is the formation of a coeloblastula from a stereoblastula by multipolar egression, the progres-sive migration of the internal cells to the periphery. Penetration of symbiotic bacteria into the embryos of all species investigated, and.
  • Coeloblastula Parenchymula Ventilation Défenses chimiques Filtreurs Cycle de calcium . Title: Mots_Cles_Porifera Author: Adam Oliver Brown Created Date: 1/13/2010 9:51:02 PM.
  • N. vectensis development proceeds through a coeloblastula stage to gastrulation, which occurs mainly by invagination 34,35. After gastrulation, the embryo emerges from the egg jelly as a free.
  • Caesar cipher Caesar cipher, is one of the simplest and most widely known encryption techniques. The transformation can be represented by aligning two alphabets, the cipher alpha
  • Small globular fossils known as Olivooides and Markuelia from basal Cambrian rocks in China and Siberia, respectively, contain directly developing embryos of metazoans. Fossilization is due to early diagenetic phosphatization. A nearly full developmental sequence of Olivooides can be observed, from late embryonic stages still within an egg membrane, to hatched specimens belonging to several.

Embryologie générale : segmentation (vue d'ensemble et

In Vaceletia cripta, Vacelet (1979) described the formation of a dense morula from the coeloblastula by means of ingression of cells from the area of one of the poles . Outwardly this process resembles unipolar immigration, well-known in many Cnidaria (Martin 1997). Usually ingresssion implies that separate cells from the blastula wall move into the blastocoel. Careful examination of semi-thin. Find the perfect ingressing stock photo. Huge collection, amazing choice, 100+ million high quality, affordable RF and RM images. No need to register, buy now Blastula of frog is coeloblastula. Ans: It has a distinct blastocoel towards animal pole. SHORT ANSWER QUESTIONS TWO MARKS QUESTIONS. 44. Write distinguishing features between spermatogenesis and oogenesis. Spermatogenesis Oogenesis *Occurs in the testis Ovary Each primary spermatocyte results in four functional sperms Each primary oocyte results in only one functional Ovum Growth phase short.

Metamorphosis of Coeloblastula Performed by Multipotential

(b) Pendant l' embryogenèse animale, au stade de la gastrulation, lorsque s'invagine le pôle végétatif de la coeloblastula, l'ouverture qui se forme par invagination est le blastopore. (c) Chez les protostomiens (némertes, némathelminthes, annélides, mollusques). Le blastopore de la gastrula deviendra la bouche de l'adulte sans. As nouns the difference between blastula and blastocyst is that blastula is (embryology) an early form in the development of an embryo, consisting of a spherical layer of cells filled with fluid a blastosphere while blastocyst is..

Coeloblastula - Wiktionar

L'embryon passe ensuite par le stade de Morula (qui est ici une coeloblastula). Gastrulation. Cette phase de l'ontogenèse aboutit à la formation des trois feuillets embryonnaires (chez les organismes triploblastiques comme l'échinoderme). Elle aboutit à la formation d'une gastrula qui possède par définition un archentéron [4] permettant le commencement de la phase suivante de. At 24-25ºC, equal, total, radial cleavage yields a wrinkled coeloblastula by 6-7 hours this undergoes egression, becoming a spherical stereoblastula by 13-14 hours during this process an acellular central yolk mass is segregated from a peripheral columnar blastoderm. The ciliated stereoblastula gastrulates by a typical diphasic invagination, beginning at 15 hours, and the gastrulating.

Coeloblastule? - Futur

Find the perfect phagosome stock photo. Huge collection, amazing choice, 100+ million high quality, affordable RF and RM images. No need to register, buy now Comment dire Girovitz Anglais? Prononciation de Girovitz à 1 prononciation audio, et de plus pour Girovitz Lisez ce Sciences et Technologies Cours et plus de 241 000 autres dissertation. Diversité du vivant. DIVI _ chapitre 2_Diploblastiques versus Triploblastiques 25/09/2017 Chapitre 2 Diploblastiques versus Triploblastiques Les Métazoaires retournent à l'état unicellulaire libre avec.. In this paper, I propose two new suborders, Refertina and Vacatina, restructured from the traditional suborders, on the basis of molecular data and the key morphological characteristics of the type genus for grouping the two clades: the presence or absence of a coeloblastula and the mode of gastrulation Abstract. Haeckel's studies of development in calcareous sponges (1872) led him to develop the Gastraea Theory, which proposes that the ancestral mode of germ layer formation, or gastrulation, was by invagination to produce a functional gut


Development of the Cardiovascular System

The circulatory system develops initially via vasculogenesis, with the arterial and venous systems developing from distinct embryonic areas.

Learning Objectives

Outline the development of the cardiovascular system

Key Takeaways

Key Points

  • The aortic arches are a series of six, paired, embryological vascular structures that give rise to several major arteries. The first and second arches disappear early. The third arch becomes the carotid artery.
  • The fourth right arch forms the right subclavian artery, while the fourth left arch forms the arch of the aorta. The fifth arch disappears on both sides.The proximal part of the sixth right arch persists as the proximal right pulmonary artery. The sixth left arch gives off the left pulmonary artery.
  • Approximately 30 posterolateral branches arise off the dorsal aortae and will form the intercostal arteries, the upper and lower extremity arteries, the lumbar arteries, and the lateral sacral arteries. The lateral branches of the aorta form the definitive renal, suprarenal, and gonadal arteries.
  • The ventral branches consist of the vitelline and umbilical arteries. The vitelline arteries form the celiac, and superior and inferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the internal iliac arteries.
  • The venous system develops from the vitelline veins, umbillical veins, and the cardinal veins, all of which empty into the sinus venosus.

Key Terms

  • sinus venosus: A large quadrangular cavity that precedes the atrium on the venous side of the chordate heart. In humans, it exists distinctly only in the embryonic heart, where it is found between the two venae cavae.
  • aortic arches: Also known as pharyngeal arch arteries, this is a series of six, paired, embryological vascular structures that give rise to several major arteries. They are ventral to the dorsal aorta.
  • cardinal vein: The precardinal veins or anterior cardinal veins contribute to the formation of the internal jugular veins and, together with the common cardinal vein, form the superior vena cava. In an anastomosis by anterior cardinal veins, the left brachiocephalic vein is produced.

Vasculogenesis

The human arterial system originates from the aortic arches and from the dorsal aortae starting from week 4 of embryonic life.
The development of the circulatory system initially occurs by the process of vasculogenesis, the formation of new blood vessels when there are no preexisting ones.

Embryonic cardiovascular system: A profile view of a human embryo estimated at twenty or twenty-one days old.

Vasculogenesis is when endothelial precursor cells (angioblasts) migrate and differentiate in response to local cues (such as growth factors and extracellular matrix) to form new blood vessels. The human arterial and venous systems develop from different embryonic areas.

Aortic Arches

The aortic arches—or pharyngeal arch arteries—are a series of six, paired, embryological vascular structures that give rise to several major arteries. They are ventral to the dorsal aorta and arise from the aortic sac.

Arches 1 and 2

Aortic arches: A schematic of the aortic arches and their arterial destinations.

The first and second arches disappear early, but the dorsal end of the second gives origin to the stapedial artery, a vessel that atrophies in humans, but persists in some mammals. It passes through the ring of the stapes and divides into supraorbital, infraorbital, and mandibular branches that follow the three divisions of the trigeminal nerve.

The infraorbital and mandibular branches arise from a common stem, the terminal part of which anastomoses with the external carotid. On the obliteration of the stapedial artery, this anastomosis enlarges and forms the internal maxillary artery the branches of the stapedial artery are now branches of this vessel.

The common stem of the infraorbital and mandibular branches passes between the two roots of the auriculotemporal nerve and becomes the middle meningeal artery. The original supraorbital branch of the stapedial artery is represented by the orbital branches of the middle meningeal artery.

Arches 3 and 4

The third aortic arch constitutes the commencement of the internal carotid artery, and is named the carotid arch. The fourth right arch forms the right subclavian artery as far as the origin of its internal mammary branch. The fourth left arch constitutes the arch of the aorta between the origin of the left carotid artery and the termination of the ductus arteriosus.

Arches 5 and 6

The fifth arch disappears on both sides.The proximal part of the sixth right arch persists as the proximal part of the right pulmonary artery, while the distal section degenerates. The sixth left arch gives off the left pulmonary artery and forms the ductus arteriosus.

This duct remains during fetal life, but closes within the first few days after birth due to increased O2 concentration. This causes the production of bradykinin which causes the ductus to constrict, occluding all flow. Within one to three months, the ductus is obliterated and becomes the ligamentum arteriosum.

Aortic Branches

The dorsal aortae are initially bilateral and then fuse to form the definitive dorsal aorta. Approximately 30 posterolateral branches arise off the aorta and will form the intercostal arteries, upper and lower extremity arteries, lumbar arteries, and the lateral sacral arteries.

The lateral branches of the aorta form the definitive renal, suprarenal, and gonadal arteries. Finally, the ventral branches of the aorta consist of the vitelline arteries and umbilical arteries.

The vitelline arteries form the celiac, and superior and inferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the internal iliac arteries.

The human venous system develops mainly from the vitelline, umbilical, and cardinal veins, all of which empty into the sinus venosus. The venous system arises during the fourth to eighth weeks of human development.

Clinical Example

Most defects of the great arteries arise as a result of the persistence of aortic arches that normally should regress or due to the regression of arches that normally should not.

A double aortic arch occurs with the development of an abnormal right aortic arch, in addition to the left aortic arch, forming a vascular ring around the trachea and esophagus, which usually causes difficulty breathing and swallowing.

Occasionally, the entire right dorsal aorta abnormally persists and the left dorsal aorta regresses. In this case, the right aorta will have to arch across from the esophagus, causing difficulty breathing or swallowing.


Types of cleavage

Determinate

Determinate is the form of cleavage in most protostomes. It results in the developmental fate of the cells being set early in the embryo development. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete embryo.

Indeterminate

A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is a characteristic of deuterostomes - when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.

Holoblastic

In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells (cells with a small even distribution of yolk) or in mesolecithal cells (moderate amount of yolk in a gradient) - bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage. [ 1 ] These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.

Meroblastic

In the presence of a large amount of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage. Two major types of meroblastic cleavage are discoidal and superficial. [ 2 ]

  • Discoidal
  • Superficial
  • Bilateral (tunicates, amphibians)
  • Radial (sea urchin, amphioxus)
  • Rotational (mammals)
  • Spiral (annelids, mollusks)
  • Discoidal (fish, birds, reptiles)
  • Superficial (insects)

PERKEMBANGAN HEWAN

Gametogenesis merupakan proses pembentukan gamet yang terjadi di dalam gonade.Proses tersebut pada hewan jantan disebut spermatogenesis yang terjadi di dalam testis, sedang pada hewan betina disebut oogenesis yang terjadi di dalam ovarium. Gametogenesis merupakan pembelahan pemasakan yaitu dengan pembelahan meiosis sehingga sel kelamin yang dibentuk bersifat haploid.

Spermatogenesis berlangsung dengan 2 tahap yaitu spermatositogenesis dan spermiogenesis (metarnorfosis). Spermatositogenesis diawali dari spermatogonium (diploid) kemudian memasuki pembelahan meiosis I sebagai spermatosit primer akan membentuk 2 spermatosit sekunder. Spermatosit sekunder mengalami pembelahan meiosis II masing-masing membentuk dua spermatid. Diferensiasi spermatid menjadi spermatozoon disebut dengan spermiogenesis. Spermatogenesis terjadi pada dinding tubulus seminiferus testis sehingga pada dinding tersebut dapat diamati berbagai
stadium perkembangan rnulai dan bagian penifer sampai ke lumen. Selain terdapat sel spermatogenik juga dapat ditemukan sel Sertoli yang berfungsi untuk memberi nutrisi bagi sperma yang terbentuk.

Gambaran struktural sel spermatogenik pada dinding tubulus seminiferus berturut-turut dan luar ke dalam sebagai berikut:
1. spermatogonium: inti oval — bulat, terpulas kuat-lemah
2. spermatosit primer: inti paling besar
3. spermatosit sekunder: inti lebih kecil, terletak dekat lumen (meiosis I)
4. spermatid: inti memanjang, melekat dekat sel Sertoli (meiosis II)
5. spermatozoa: sel berekor yang menjulur ke lumen. (Gambar 1)

Spermiogenesis berlangsung melalui 3 tahap yaitu:
1. pembentukan tudung kepala (akrosom) yang berasal dari badan Golgi
2. pembentukan keping tengah dan flagela, yang berasal dari sentriol dan bagian ini letaknya berseberangan dengan letak akrosom terhadap inti.
3. pemanjangan inti, pengurangan sitoplasma dan migrasi mitokondria menuju keping tengah.

Spermatogenesis dirangsang oleh FSH, sedangkan LH (ICSH) merangsang sel Leydig (sel yang terdapat diantara tubulus seminiferus) sehingga menghasilkan hormon testoteron.
Oogenesis berlangsung didalam ovarium dan sel telur diselaputi oleh sel folikel sehingga membentuk folikel ovarium. Berbeda dengan sperma, pembelahan pemasakan (meiosis) dan 1 oogonium hanya menghasilkanl sel telur (ovum) sebab selama pembelahan akan terbentuk badan polar (polosit).
Oosit primer dari oogonium sesudah meiosis I akan membentuk 1 oosit sekunder dan 1 badan polar. Bersamaan dengan pembelahan pemasakan tersebut juga terjadi pertumbuhan folikel ovarium sehingga terbentuk folikel primer, folikel sekunder, folikel tertier sampai folikel masak (folikel Graaf). Folikel Graaf (stadium oosit sekunder) kemudian mengalami ovulasi sehingga sel telur keluar dan ovarium menuju ke oviduct.

Folikel yang ditinggalkan oleh sel telur kemudian akan membentuk corpus luteum yang menghasilkan hormon progesteron. Sel-sel folikel selama dalam pertumbuhannya dapat menghasilkan hormon estrogen. Pertumbuhan dan perkembangan folikel dirangsang oleh FSH, sedangkan proses ovulasi dirangsang oleh LH.

Selaput pembungkus telur meliputi:
1. membran primer: dibentuk oleh ooplasma sebagai membran vitelin (oolemma)
2. membran sekunder: dibentuk oleh sel folikel sebagai zona pelusida, corona radiata, sel granulosa, sel theca.
3. selaput tertier: dibentuk di dalam oviduct sebagai lendir (pada katak), khonon (pada ikan), albumen (pada burung).
4. selaput quarter: dibentuk di dalam uterus sebagai kulit kapur (pada burung), kapsula (pada ikan), khitin (pada seranga).
Sel telur mempunyai sumbu yang berorientasi pada sumbu animal dan sumbu vegetal, yaitu sebagai polus animalis (dorsal) dan polus vegetativus (ventral). Polus animalis berisi ooplasma, polus vegetativus berisi deutoplasma (vitelus/yolk). Vitelus/yolk disintesis di dalam hepar dan masuk ke dalam telur melalui pembuluh darah. Atas dasar keadaan vitelus tersebut, maka sel telur dapat dibagi menjadi
I. Berdasarkan penyebaran vitelus:
1. isolesital/homolesital: tersebar merata
2. telolesital: tersebar pada salah satu ujung
3. sentrolesital: tersebar di bagian tengah
II. Berdasarkanjumlah vitelus:
1. oligolesital: vitelus sedikit
2. mesolesital: vitelus sedang
3. polilesital: vitelus banyak
4. megalesital: vitelus lebih banyak (sangat banyak).

Fertilisasi (pembuahan) merupakan proses peleburan (penggabungan) inti sel telur dan inti sperma. Fertilisasi dapat terjadi secara interna yaitu yang terjadi di dalam oviduct (dalam tubuh induk) atau secara externa yaitu yang terjadi di air (di luar tubuh induk). Selama dalam perjalanannya di dalam saluran reproduksi maka sel kelamin akan mengalami pemasakan. Fertilisasi merupakan proses yang bertahap yaitu:
1. Tahap persiapan yaitu proses pemasakan sel kelamin
2. Tahap penempelan yaitu saat menyentuh selaput telur
3. Tahap penetrasi yaitu proses penyusupan sperma ke dalam sel telur. Penetrasi terjadi setelah membran telur larut karena pengaruh enzim hialuronidase yang dikeluarkan oleh akrosom.
4. Tahap peleburan/penggabungan inti sperma dan inti telur sehingga terbentuk zygot yang diploid (sesuai dengan individu yang mewariskan)
5. Tahap awal perkembangan yang merupakan rangsangan (triger) agar terjadi perkembangan

Fertilisasi umumnya bersifat monospermi (satu inti sperma yang membuahi) agar individu yang terbentuk diploid dan selalu dihindari agar tidak bersifat polispermi (lebih dari satu inti sperma yang membuahi). Hal tersebut disebabkan oleh karena pada saat fertilisasi terjadi aktivasi granula cortex di dalam sel telur sehingga permeabilitas membran telur berubah. Perbedaan permeabilitas membran tersebut menyebabkan sperma lain tidak dapat menembus sel telur. Pada Vertebrata setelah terjadi fertilisasi, ada yang kemudian meletakkan telur di luar tubuh sehingga telur-telur tersebut akan
diinkubasikan. Hewan demikian dikatakan bersifat vivipar. Sedangkan pada Vertebrata lain telur yang difertilisasi akan berkembang di dalam tubuh induk (selama masa kehamilan) dan kemudian akan melahirkannya. Kelompok hewan demikian dikatakan bersifat vivipar. Perkembangan di dalam uterus diawali dengan adanya implantasi (penempelan pada dinding uterus) sehingga terjadi kehamilan. Setelah terjadi implantasi, jaringan embrio dan jaringan induk akan membentuk plasenta.

B. Segmentasi dan Balstula

Pembelahan zygot terjadi secara mitosis yang berlangsung sangat cepat tidak terjadi pertumbuhan mulai dari sel tunggal menjadi masa sel yang padat disebut morula, Masing-masing sel dari pembelahan awal tersebut dikenal sebagai blastomer, Pembelahan terjadi melalui bidang-bidang pembelahan yaitu :
1. Bidang meridional : bidang tegak melalui polus animalis (PA) dan polus vegetativus (PV)
2. Bidang ekutorial : bidang datar diantara PA dan PV
3. Bidang sagital : bidang yang membagi bagian kanan dan bagian kiri
4. Bidang latitudinal : bidang datar yang terletak diantara bidang ekuatorial dengan PA dan PV
5. Bidang transversal : bidang tegak lurus bidang ekuatorial

Pembelahan awal (I) dan II melalui bidang meridional, sedang pembelahan III melalui bidang ekuatorial.
Jenis-jenis pembelahan:
1. Holoblastik: pembelahan terjadi pada semua bagian yang biasanya terjadi pada telur yang isolesital atau telolesital sedang contoh pada Amphioxus, Amphibia. Holoblastik ada yang radial (sea urchin), bilateral (Amphibia), spiral (molusca) rotational (mamal)
2. Meroblastik: pembelahan terjadi hanya pada bioplasma (daerah animalis), bagian
deutoplasma tidak membelah meroblastik ada 2 macam yaitu meroblastik discordal (pada burung, reptil) dan superfisial (pada serangga). Pembelahan ini umumnya terjadi pada telur yang
sentrolesital dan telolesital berat (Gambar 1. dan Gambar 2.)
Setelah terjadi pembelahan yang cepat sampai terbentuk morula yang padat, maka pembelahan selanjutnya akan membentuk rongga disebut blastocoel. Dinding rongga tersebut terdiri sel-sel (blastomer) yaitu sebagai sel formatif pembentuk badan embrio dan sel auxilary pembentuk selaput embrio. Blastomer di daerah animal lebih kecil (mikromer) dan pada di daerah vegetal (makromer). Berdasarkan bentuknya blastula ada yang bulat (blastosphere), pipih/cakram (discoblastula) dan gelembung (blastocyst). Sedangkan atas dasar strukturnya maka terdapat blastula berongga (coeloblastula), blastula masif (stereoblastula), blastula dengan lapisan sel (blastoderm).

Beberapa contoh:
1. Pada ikan: pembelahan terjadi secara holoblastik, meskipun pada daerah vegetal lebih lambat. Blastema pada polus vegetativus relatif lebih besar sebabyolk lebih banyak, sedang pada polus animalis lebih kecil dan membentuk blastoderm. Blastula bertipe discoblastula dengan rongga
yang relatif sempit. Blastoderm ada yang membentuk blastodisc.
Periblast merupakan kelompok sel yang membentuk lapisan sinsitial yang menyelubungi yolk yang tidak ikut membelah. Periblast berfungsi membantu memobilisasi yolk untuk pertumbuhan embrio.
2. Pada Amphibia: pembelahan terjadi secara holoblatik, blatomer pada polus animalis membelah lebih cepat dari pada polus vegetativus karena yolk lebih banyak pada polus vegetativus. Blastocoel letak eksentrik (mendekati polus animalis). Di daerah ekuatorial blatomer membentuk “germ ring”.
3. Pada Reptil dan Ayes: pembelahan terjadi secara meroblastik discordal karena yolk lebih banyak. Blastoderm (disebut juga blastodisc) terpisah dengan yolk. Blastoderm terpisah dari yolk oleh rongga subgerminal. Blastula bertipe discoblastula, dengan rongga pipih. Blastomer pada bagian dorsal
blastocoel disebut epiblast, pada bagian lateral disebut periblast dan pada bagian ventral disebut hypoblast.
4. Pada Mamal: stadium blastula pada mamal disebut blastocyst, dengan rongga bulat. Blastoderm akan membentuk “inner cell mass” (1CM) yang kemudian akan menjadi embrio dan diluar yolk akan membentuk tropoblast yang akan menjadi selaput extraembrional (membran choriovitelus dan
chorioalantois). (Gambar 3.) Pada akhir stadium blastula, daerah-daerah tertentu akan menjadi calon pembentuk organ tertentu (dikenal sebagai peta nasib). Pada blastula katak daerah-daerah
tersebut ialah:
- epidermal sebagai calon kulit
- neuroektodermal sebagai calon sistem saraf
- lamina prechordalis sebagai calon kepala
- chorda mesoderm sebagai calon chorda dorsalis
- mesodermal sebagai calon somit
- endodermal sebagai calon sistem pencemaan
- germinal sebagai calon gonade.

Pada daerah-daerah tersebut diatas mempunyai potensi untuk membentuk
jaringan/organ. (Gambar 4.)

C. Gastrula, Neurulasi dan Selaput Embrio

Gastrulasi merupakan stadium perkembangan yang sel-selnya membentuk lapisan lembaga (germ layer) yang terdapat di seki tar tubulus endodermal (usus primitif). Ruang tertutup di dalam usus primitif tersebut dikenal dengan gastrocoel atau archenteron. Neurulasi merupakan proses pembentukan tubulus ektodermal (canalis neuralis). Di dalam tubulus ini terdapat ruangan yang disebut neurocoel. Gastrulasi dan neurulasi sebagian besar merupakan proses penyusunan kembali sel-sel blastula di dalam embrio. Selama proses tersebut, 3 lapisan lembaga akan terbentuk yang merupakan ciri khas primi tif dan mesoderm terdapat diantara 2 lapisan tersebut. Selain gastrula mempunyai 3 lapisan (triploblastik) tersebut yang umumnya pada Vertebrata, juga terdapat gastrula dengan 2 lapisan (diploblastik) yang terdapat pada sea urchin. Pada pembelahan ditandai dengan pembelahan sel, dan pada gastrulasi ditandai dengan penyusunan kembali seluruh sel yaitu dengan terjadi gerakan sel (gerakan morfogenesis). Gerakan sel dapat berlangsung dipermukaan embrio (epiboli) dan dapat pula berlangsung di dalam embrio (emboli). Epiboli meliputi ektensi (melebar) dan elongasi (memanjang). Sedang emboli meliputi invaginasi (melekuk), evaginasi (menonjol), involusi (melekuk dan memutar), ingresi (muncul dan lapisan), convergensi (menyempit), divergensi (melebar), delaminasi (tergeser dan sekitamya) dan intercalasi (terdesak). (Gambar 1 . dan Gambar 2.)

Gerakan sel kearah dalam diawali oleh sel permukaan yaitu ektoderm. Pada permukaan, ektoderm menebal menjadi lempeng neural yang memanjang pada sisi dorsal sumbu anterior posterior embrio. Pada bagian tepi lempeng neural akan tumbuh ke dorsal membentuk lipatan neural. Lempeng neural akan bertemu dan bergabung pada bagian dorsal membentuk canalis neuralis yang menyelubungi neurocoel. Canalis neuralis akan terdiferensiasi membentuk encephalon dan medulla spinalis.
Selama lipatan neural bergabung, beberapa sel pada lipatan ektoderm memisahkan diri membentuk kelompok sel disebut neural crest. Endoderm merupakan derivat sel-sel yang bergerak masuk dan permukaan luar blastula. Endoderm yang pertama kali terbentuk membentuk dinding usus yang yang terbentang dari anterior sampai posterior embrio. Mesoderm juga merupakan derivat dan sel-sel yang bergerak dari permukaan luar blastula. Proliferasi sel-sel mesodermal akan menyebar masuk jaringan di dalam tubuh diantara ektoderm luar dan endoderm dalam. (Gambar 3.)

Selaput extraembrional berasal dari tropoblast. Selaput extraembrional meliputi kantong yolk, amnion, khorion dan allantois. Amnion, khorion dan allantois dijumpai hanya pada reptil, ayes dan mamal. Kantong yolk kebanyakan pada yang masih primitif. Kantong yolk mengitari yolk, kemudian kosong menjadi usus tengah dan dilapisi oleh endoderm. Kantong yolk kaya vascularisasi melalui arteria vitelina dan vena vitelina. Tetes-tetes yolk di dalam kantong selalu dicerna oleh enzim yang
disekresi oleh lapisan pada kantong yolk dan dibawa ke embno melalui vena vitelina.
Kantong yolk mengecil seiring dengan pertumbuhan embrio. (Gambar 4.)
Pada embrio reptil, ayes dan mamal berkembang 2 selaput yaitu amnion dan khorion.
Kedua selaput tersebut dibentuk pada saat pelipatan ke dorsal dari somatopleura dan kemudian akan saling bertemu membentuk amnion yang mengelilingi embnio. Khonion dibentuk dari pertumbuhan somatopleura yang mengelilingi amnion dan kantong yolk. Di dalam amnion berisi cairan amnion yang mengandung air metabolik dari jaringan embrio. (Gambar 5.)
Allantois merupakan suatu kantong extraembrional yang berkembang dari proses evaginasi bagian ventral cloaca. Perkembangan berikutnya akan menempel pada permukaan dalam khorion membentuk selaput khorioalantois. Pada mamal selaput khorioallantois menempel langsung pada dinding uterus sehingga membentuk plasenta khorioallantois yang berfungsi untuk membawa nutrisi dari induk ke embrio dan membawa sisa matabolisme dari embrio ke induk. Pada amniota, sebagian besar
allantois yang berbatasan langsung dengan bagian proximal cloaca akan menjadi vesica urinaria, sedang dengan bagian distal membentuk urachus. Plasenta merupakan organ pada hewan vivipar dimana jaringan induk dan jaringan embrio berkaitan sangat erat dan saling mengalirkan diantara induk dengan embrio.
Plasenta terdiri:
1. selaput extraembrional (kantong yolk, selaput khoriovetelina, selaput khorioallantois atau khorion).
2. berkaitan erat dengan dinding uterus induk yang kaya vascularisasi.

Pada marsupialia, dan kebanyakan ungulata, selaput extraembrional tidak melekat erat dengan dinding uterus sehingga pada waktu kelahiran mudah lepas, keadaan ini dikenal dengan plasenta nondesiduata. Sedangkan plasenta desiduata, jika villi khorion melekat erat dan terinvasi ke dalam jaringan uterus sehingga pada saat kelahiran terjadi pendarahan. Atas dasar penyebaran villi khorion pada permukaan kantong khorion maka ada beberapa macam plasenta antara lain: plasenta
kotiledonaria (villi terkumpul sebagai bercak-bercak) pada kuda & sapi, plasenta zonaria (villi berbentuk seperti sabuk) pada kucing & anjing, plasenta diskoidal (villi berbentuk seperti cakram) pada beruang & manusia, plasenta difusa (villi tersebar merata) pada babi. (Gambar 6.)


See also

An embryo is the early stage of development of a multicellular organism. In general, in organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization and continues through the formation of body structures, such as tissues and organs. Each embryo starts development as a zygote, a single cell resulting from the fusion of gametes. In the first stages of embryonic development, a single-celled zygote undergoes many rapid cell divisions, called cleavage, to form a blastula, which looks similar to a ball of cells. Next, the cells in a blastula-stage embryo start rearranging themselves into layers in a process called gastrulation. These layers will each give rise to different parts of the developing multicellular organism, such as the nervous system, connective tissue, and organs.

Hemichordata is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The extinct class Graptolithina is closely related to the pterobranchs.

Embryology is the branch of biology that studies the prenatal development of gametes, fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.

Blastulation is the stage in early animal embryonic development that produces the blastula. The blastula (from Greek βλαστός is a hollow sphere of cells surrounding an inner fluid-filled cavity. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

In developmental biology, gastrulation is a phase early in the embryonic development of most animals, during which the blastula is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.

An egg is the organic vessel containing the zygote in which an embryo develops until it can survive on its own, at which point the animal hatches. An egg results from fertilization of an egg cell. Most arthropods, vertebrates, and mollusks lay eggs, although some, such as scorpions, do not.

In biology, a blastomere is a type of cell produced by cleavage of the zygote after fertilization and is an essential part of blastula formation.

A blastocoel, also spelled blastocoele and blastocele, and also called blastocyst cavity is a fluid-filled cavity that forms in the blastula (blastocyst) of early amphibian and echinoderm embryos, or between the epiblast and hypoblast of avian, reptilian, and mammalian blastoderm-stage embryos.

In developmental biology, embryonic development, also known as embryogenesis, is the development of an animal or plant embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

Isolecithal refers to the even distribution of yolk in the cytoplasm of ova of mammals and other vertebrates, notably fishes of the families Petromyzontidae, Amiidae, and Lepisosteidae. Isolecithal cells have two equal hemispheres of yolk. However, during cellular development, normally under the influence of gravity, some of the yolk settles to the bottom of the egg, producing an uneven distribution of yolky hemispheres. Such uneven cells are known as telolecithal and are common where there is sufficient yolk mass.

Centrolecithal describes the placement of the yolk in the centre of the cytoplasm of ova. Many arthropod eggs are centrolecithal.

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.

In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus. This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred. The ICM lies within the blastocoele and is entirely surrounded by the single layer of cells called trophoblast.

An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to symmetric cell divisions which give rise to daughter cells of equivalent fates. Notably, stem cells divide asymmetrically to give rise to two distinct daughter cells: one copy of the original stem cell as well as a second daughter programmed to differentiate into a non-stem cell fate.

Epiboly describes one of the five major types of cell movements that occur in the Gastrulation stage of embryonic development of some organisms. Epibolic movement is the way in which a layer epithelial cells spreads. This can be achieved in multiple ways.

In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.

The development of fishes is unique in some specific aspects compared to the development of other animals.

Capitella teleta is a small, cosmopolitan, segmented annelid worm. It is a well-studied invertebrate, which has been cultured for use in laboratories for over 30 years. C. teleta is the first marine polychaete to have its genome sequenced.

Leech embryogenesis is the process by which the embryo of the leech forms and develops. The embryonic development of the larva occurs as a series of stages. During stage 1, the first cleavage occurs, which gives rise to an AB and a CD blastomere, and is in the interphase of this cell division when a yolk-free cytoplasm called teloplasm is formed. The teloplasm is known to be a determinant for the specification of the D cell fate. In stage 3, during the second cleavage, an unequal division occurs in the CD blastomere. As a consequence, it creates a large D cell on the left and a smaller C cell to the right. This unequal division process is dependent on actomyosin, and by the end of stage 3 the AB cell divides. On stage 4 of development, the micromeres and teloblast stem cells are formed and subsequently, the D quadrant divides to form the DM and the DNOPQ teloblast precursor cells. By the end stage 6, the zygote contains a set of 25 micromeres, 3 macromeres and 10 teloblasts derived from the D quadrant.


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