Introduction.                                                                                                                                                           

All events which change a zygote or a blastos (a reproductive unit in asexual reproduction e.g., whole organism e.g. Amoeba, a bud e.g. Hydra, or a body fragment e.g. sea anemone) into a fully developed organism is called development. The entire process of development, which passes through embryo formation is called embryogenesis. Von Baer is commonly called “The father of modern embryology.”

 Embryonic development.                                                                                                                                   

It includes a definite series of phases which are fundamentally similar in all sexually reproducing organisms and transform a one-celled zygote to a multicellular and fully formed development stage till hatching or birth. It is divided into following types.

1. Pre natal or embryonic period : It is the period of development from the diploid one-celled zygote to a multicellular embryo. It occurs either inside the egg or mother’s womb and extends upto hatching or birth. The study of the changes during this period is called embryology.
2. Post natal or post embryonic period : It is the period of development which extend from hatching or birth to death. The branch of science which deals with the study of progressive, orderly and gradual changes in structure and functioning of organism during entire life history from zygote or blastos to death, is called development biology.

 Phases of embryonic development.                                                                                                                

Embryonic development involves following dynamic changes and identifiable process.

1. Gametogenesis : It involve the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary).

It is of two types

1. Spermatogenesis : Formation of sperm.           (b) Oogenesis : Formation of ova

2. Fertilization : It involve the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis.
3. Cleavage : It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation.
4. Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.
5. Gastrulation : It includes the mass and orderly migration of the organ specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers.
6. Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.

Important Tips

Historical background of Embryonic Development :

• George Newport : Observed fertilization of frog’s egg.
• Oscar Hertwig : Described the fusion of sperm and egg nuclei in sea urchin.
• Prevost and Dumas : Reported cleavage of frog’s egg.
• Swammerdam : Observed the first cleavage of frog in 1738.
• Spallanzani : Detailed process of cleavage of frog’s egg.
• Von Bear : Proposed recapitulation theory.
• Ernst Haeckel : Modified recapitulation theory to “Biogenetic law”. It states “Ontogeny repeats phylogeny.”
• H. Spemann and Mangold : Reported embryonic induction on newt and gave concept of primary organizers.
• Pander : Formation of three germinal layers in chick embryo.
• Oviparity : Fertilization may be external or internal but development always outside the female body and inside the egg e.g. most of non-chordates, fishes, amphibians and reptiles; all birds and prototherians.
• Ovoviviparity : Fetilization always internal. Development also inside the uterus and baby is born but there is no placenta formation so egg is yolky e.g. rattele snake, Dog fish
• Viviparity : Fertilization and development always inside the body. Placenta is formed and female gives birth to young one e.g. most of mammals.

 Fertilization.                                                                                                                                                            

1. Definition : Fusion of a haploid male gamete (spermatozoon) and a haploid female gamete (ovum) to form a diploid cell, the zygote, is called fertilization or syngamy.
2. Site of fertilization : Fertilization in human female is internal as in other mammals. It takes place usually in the ampulla of the fallopian tube.

(iii)Steps of fertilization

1. Approach of sperm to ovum : Male discharge semen (3.5 ml) high up in the female’s vagina, close to the cervix during coitus. This is called ejaculation or insemination. This ejaculation contains as many as 400 million sperms but only about 100 sperms reach the fallopian tube because many sperms are either killed by the acidity of female genital tract or engulfed by the phagocytes of the vaginal epithelium. The sperm swim in the seminal fluid at the rate of 1-4 mm per minute by the aspiratory action of the uterus and peristaltic movement of the fallopian tube.

Capacitation is the phenomenon of physiological maturation of sperms by breaking of acrosome membrane inside the female genital tract. It takes about 5-6 hours. Ovum is released on the 14^th day of menestrual cycle trapped by the fimbriae of the ampulla of fallopian tube and move towards the uterus by peristalsis and ciliary action. At the time of ovulation, egg is at secondary oocyte stage. Fertilizability of human sperm in the female genital tract is of 12 to 24 hours while its survival value is upto 3 days and of ovum is only 24 hours though it can live for about 72 hours.

2. Penetration of sperm : The ovum secretes a chemical substance called fertilizin, which has a number of spermophillic sites on its surface where the sperm of species specific type can be bound by their antifertilizin site. This fertilizin-antifertilizin interaction, causing agglutination (sticking together) of egg and sperm.

The sperm generally comes in contact with ovum in the animal pole (side of ovum with excentric nucleus) while the opposite side of ovum is called vegetal pole. Ovulation in the human female occurs at secondary oocyte stage in which meiosis-I has been completed and first polar body has been released but second maturation is yet to complete. Penetration of sperm is a chemical mechanism. In this acrosome of sperm undergoes acrosomal reaction and releases certain sperm lysins which dissolve the egg envelopes locally and make the path for the penetration of sperm. Sperm lysins are acidic proteins. These sperm lysins contain a lysing enzyme hyaluronidase which dissolves

the hyaluronic acid polymers in the intercellular spaces which holds

the granulosa cells of corona radiata together; corona penetrating enzyme (that dissolves the corona radiata) and acrosin (which dissolves the zona pellucida). Then it dissolves the zona pellucida. Only sperm nucleus and middle piece enter the ovum. The tail is lost.

3. Cortical reaction : Immediately after the entry of a sperm into the egg, the later shows a cortical reaction to check the entry of more sperms. In this reaction, the cortical granules present beneath the egg’s plasma membrane release chemical substance between the

Penetration path

Copulation path

Polar bodies

Gray crescent Fertilization membrane

ooplasm and the plasma membrane (vitelline membrane). These substances raise the vitelline membrane above the egg surface. The

Fig : Penetration and copulation paths of the sperm nucleus in frog egg during fertilization

elevated vitelline membrane is called fertilization membrane. The increased space between the ooplasm and the fertilization membrane and the chemical present in it effectively check the entry of other sperm. If polyspermy occurs, that is more than one sperm enter the secondary oocyte, the resulting cell has too much genetic material to develop normally.

Sperm penetration into ovum also induces following metabolic activities :

1. The egg surface produces fertilization cone.

Corona radiata

1

2

Zona pellucida

Dispersal of corona cells

2. The vitelline membrane is lifted and is converted into fertilization membrane.
3. The cortical granule explode.
4. The cytoplasm exhibits movements.
5. The permeability of plasma membrane increases.
6. The coenzyme NAD is phosphorylated.
7. The rate of protein synthesis increases.
8. Mitosis is initiated.
9. The breakdown of polysaccharide occurs.

Zona pellucida Oolemma

3

4

5

6

7                         8

Amphimixis            Zygote

10. The enzyme dehydrogenase increases.

Fig. Steps of fertilization

4. Fusion of gametic nuclei : Entrance of spermatozoon serves to acts as stimulus which causes the second maturation division. As the head and middle piece of the sperm advance into the egg, those parts rotate through an angle of 180° so that the mitochondria and proximal centriole of the associated middle piece assume the leading position. Beside this rotation, the chromatin itself starts swelling by absorbing fluid from the surrounding cytoplasm and becomes vesicular. It is now called male pronucleus. This direction of movement of male pronucleus is called penetration path. The centriole brought in by the spermatozoon subdivides into two and as achromatic spindle is established in the center of the active cytoplasm. With the production of the second polar body, the egg nucleus or female pronucleus is ready for union with the male pronucleus provided by the sperm head.

The male pronucleus which has been advancing the penetration path, now moves directly toward the female pronucleus. This in many cases involves a slight change in the course of sperm. In such cases, the later portion of its course is called the copulation path. The centrioles of middle piece of sperm form a spindle. The nuclear membrane of the gametic nuclei degenerates and two sets of chromosomes initially lie on two poles of the spindle but later these sets of chromosomes mix up and the process is called amphimixis. The fertilized egg is now called zygote and the zygote nucleus is called synkaryon.

(iv)Types of fertilization

1. External fertilization : In this, the gamete fuse outside the female body and is found in most of bony fishes (e.g. Labeo), amphibians (e.g. frog), all echinoderms (e.g. starfish) and lower chordates (e.g. Herdmania).
2. Internal fertilization : In this, the fusion of gametes in some part of female genital tract and generally near the ostium. It is found in all terrestrial animals which may be oviparous (all birds, prototherians), ovo- viviparous (rattle-snake) or viviparous (all marsupials and eutherians).
3. Self fertilization (Endogamy) : In this, two fusing gametes are derived from the same parent (uniparental) e.g. Taenia, Fasciola (sheep, liver fluke).
4. Cross fertilization (Exogamy) : In this, two fusing gametes are derived from different parents (biparental). It is found in all unisexual animals and some bisexual animals e.g. Pheretima (earthworm-due to protandry), Scypha (Sycon-due to protogyny) Fasciola and Taenia (have both self and cross fertilization).
5. Monospermic fertilization : When only one sperm enters and fuses with ovum. It is found in most of animals.
6. Polyspermic fertilization : When many sperms penetrate the ovum and may be pathological polyspermy (due to over-ripening of egg) or physiological polyspermy (natural entry of sperms). But only one sperm fuses with ovum.

(v)Significance of fertilization

1. It provides stimulus for the egg to complete its maturation.
2. It activates the ovum to develop into a new individual by repeated mitotic division.
3. Fertilization restores the diploid number of chromosomes (46 in man) in the zygote by adding male’s haploid set of chromosomes.
4. It makes the egg more active metabolically.
5. It combines the character of two parents and introduces variations. So help in evolution.

6. Sex chromosomes of sperm is either X or Y and helps in sex determination.
7. Fertilization membrane formed after sperm entry, checks the entry of additional sperms.
8. Copulation path sets the axis of division.

Important Tips

• Termones : Chemical released by algae in water for attraction of gametes.
• Pheromones : Chemical released by insects in air and generally acts as sex attractants e.g. in gypsy moth.
• Gamones : Chemical released by the human gametes for their attraction.
• Zygote is called the first cell of next generation.
• Isogamy : When two fusing gametes are morphologically and physiologically similar e.g. monocystis.
• Anisogamy : When two fusing gametes are morphologically and physiologically different e.g. frog, human beings etc.
• Twins : When 2 or more babies are born in multiple births then these are called twins. These may be identical twins (or monozygotic twins) or fraternal (or dizygotic or non identical twins). Identical twins are attached to same placenta while fraternal twins are attached to uterine epithelium by separate placentae.
• Siamese twins : Conjoined twins joined at the hip, chest, back, face etc. these are surgically separated (first time in siam) and are always monozgotic.
• Free martins : A sexually under-developed female calf joined with a male.
• Polyspermy : Penetration of many sperms into an ovum simultaneously. Only one of the spermatozoa will be successful in uniting with female pronuclei.
• Polygyny : When two female pronuclei unite with a male pronucleus.
• Polyandry : Conjugation of two or more male pronuclei with a female pronucleus.
• Gynogenesis : Activation of egg by sperm, but there is no fusion of its pronucleus.
• Androgenesis : Non-participation of female pronucleus in fertilization.
• Cone of reception (Fertilization Cone) : A conical outgrowth given by egg of frog to receive the sperm. Not found in human egg.
• Fertilizin is a glycoproteinous or mucopolysaccharide molecule, while antifertilizin is a proteinaceous substance of acidic amino acids on the surface of head of sperm.
• Fertilizin-Antifertilizin reaction was proposed by F.R. Lillie
• Sperms swim in the seminal fluid at the rate of 1-4 mm per minute and time taken by the sperm entry into the oocyte is about 30 minutes.
• The slow block to polyspermy develops, in response to the formation of the fertilization membrane and within a minute after the fast block.
• The motion of sperm is Random.
• Polyspermy is of common occurance in birds.
• Bindin is a protein in acrosome which ensure that the egg is being fertilized by a sperm of the same species.

  Cleavage.                                                                                                                                                               

1. Definition : The term cleavage refers to a series of rapid mitotic division of the zygote following fertilization, forming a many celled blastula. The cleavage follows fertilization and ends with the formation of a characteristic development stage called blastula.

2. Cleavage versus typical mitosis : The cleavage division are no doubt mitotic as they produce diploid cells, they differ from typical mitosis in a couple of significant points.

3. Planes of cleavage : The cleavage is initiated by the appearance of a constriction or groove called cleavage furrow. The cleavage furrows may divided the egg from different angles or planes. These are four important planes of cleavage. They are as follows.

1. Meridional plane : When cleavage furrow bisects both the poles of the egg, passing through the animal vegetal axis, the plane of cleavage is called meridional plane.

Example : I^st and II^nd cleavage furrow of frog and I^st cleavage furrow of chick.

2. Vertical plane : When cleavage furrow passes from the animal pole to the vegetal pole, but it does not pass through the median axis of the egg.

Example : III^rd cleavage furrow of chick.

3. Equatorial plane : When cleavage furrow bisect the egg at right angles to the median axis and half way between the animal and vegetal poles.

Example : I^st cleavage plane of eggs of higher mammals.

4. Latitudinal or transverse or horizontal plane : The transverse plane resemble the equatorial plane, but it passes either above (towards the animal pole) or below (towards the vegetal pole) the equator of the egg.

Example : III^rd cleavage plane of Amphioxus and frog.

Animal pole

Median axis

Vegetal pole

A                                   B                                C                             D

Fig : (A) Meridional plane; (B)Vertical plane; (C) Equatorial plane; (D) Latitudinal plane

4. Patterns of cleavage : During segmentation, the cleavage furrows are not formed at random but are oriented in a particular manner with reference to the main (animal-vegetal) axis of the egg. The orientation of successive cleavage furrows with respect to each other and to the main axis of the egg is, however, unlike in different species. As such various patterns of cleavage are found among animals. Based upon symmetry, four patterns of cleavage have been recognized. They are as follows

1. Radial cleavage : In this cleavage pattern, division take place in such a manner that all the blastomeres are placed in a radially symmetrical fashion around the polar axis. When such an egg is viewed from the poles, the blastomeres seem to be arranged in a radially symmetric form.

Example : Sponges, coelenterates, sea urchin, sea cucumber, amphioxus.

Two-cell stage          Four-cell stage         Eight-cell stage

Fig : Radial cleavage in  sea-cucumber  Synapta digitata

Blastula (V.S)

2. Biradial cleavage : In this pattern four blastomeres arise by the usual two meridional cleavages. The third cleavage plane is vertical resulting in the formation of a curved plate of 8

cells arranged in two rows of 4 each. In these rows, the central cells are larger than the end ones.

Example : Ctenophores like Beroe.

Fig : Biradial (dorsal view)

3. Spiral cleavage : The spiral cleavage is diagonal to the polar axis. In this type, the spindles for the third cleavage, instead of being erect, are oriented diagonally so that the

resulting upper tier of cells is sidewise. The upper 4 cells are placed over the junction between the four lower cells. The upper smaller cells are called micro and lower larger cells are known as macromeres. The spiral cleavage results due to oblique positions of the mitotic spindles. This type

of cleavage is called the spiral type because the four spindle during the

third cleavage are arranged in a sort of spiral.

Examples : Eggs of annelids, molluscs, nemerteans and some of the planarians.

Fig : Spiral

4. Bilateral cleavage : In this pattern of cleavage, the blastomeres are so arranged that the right and left sides becomes distinct. In this case, two of the first four blastomeres may be larger than the other two, thus establishing a plane of bilateral symmetry in the developing embryo.

Examples : Nematodes, cephalopodes, molluscs, some echinoderms and tunicates.

Presumptive                      Slaty gray cytoplasm                notochord

Yellow cytoplasm

Mesenchyme cells

Fig : Bilateral cleavage

Presumptive plate

Muscle cells

5. Cleavage on the basis of potency : According to potentialities of early blastomeres, cleavage may be of following types.

1. Determinate cleavage or mosaic cleavage : In determinate cleavage, each early blastomere is destined to become a particular portion of embryo.

Examples : Ascaris, annelids, molluscs, ascidians, polyclads (platyhelminthes) and nemerteans.

2. Indeterminate or regulative cleavage : In contrast, early blastomeres are equivalent in their potentialities. If separated, each will give rise to a complete normal embryo.

Example : All chordates, echinoderms and arthropods.

6. Types of cleavage : The amount of yolk (Lecithality) also determines the type of cleavage. Which are as follows

1. Holoblastic cleavage : Alecithal, homolecithal and mesolecithal eggs show rapid and complete division of zygote are called total or holoblastic cleavage. Resulting 8 blastomeres after the third cleavage may be equal or unequal to each other. Accordingly they are of two types
   1. Equal holoblastic cleavage : If the blastomeres are approximately equal, it is called equal holoblastic cleavage.

Examples : Echinoderms, amphioxus and placental mammals.

2. Unequal holoblastic cleavage : If the upper 4 blastomere are smaller (micromeres) than the lower 4 yolk-laden larger blastomere (macromere), it is calld unequal holoblastic cleavage.

Example : Fish and amphibians.

Right

Dorsal

Ventral

Left

blastomere

blastomere blastomere

Second furrow

Zonal pellucida Polar bodies

Zygote

Zona pellucida Blastomeres

blastomere

Animal hemisphere

First furrow

Albumen                      Albumen

Zygote                   2-cell stage                          Morula

Fig : Holoblastic equal cleavage

Grey crescent

Vegetal hemisphere

Fig : Holoblastic unequal cleavage

2. Meroblastic cleavage : In large polylecithal eggs cleavage furrow cannot cut through the enormous yolk present so that the entire egg is not divided into cells. Thus cleavage is incomplete or partial, termed meroblastic. It is of following two types
   1. Discoidal cleavage : Cleavage are restricted only to the small cytoplasmic cap at the animal pole resulting in a rounded embryonic or germinal disc is termed discoidal cleavage.

Example : Eggs of elasmobranchs, bony fishes, birds, reptiles and egg laying mammals.

2. Superficial cleavage : Cleavage is restricted to a superficial peripheral layer of cytoplasm around yolk, hence the term superficial cleavage.

Example : Centrolecithal eggs of arthropods.

B                     A

Yolk

Egg with much yolk (reptiles, birds, most fishes; some invertebrates as the squid)

Yolk          Yolk

Central yolk mass (most arthropods)                                                          A

Fig : Types of cleavage and the resulting blastulae and gastrulae

7. Cleavage in human zygote : Cleavage in the human zygote occurs during its passage through the fallopian tube to the uterus as in other mammals. It is holoblastic. The first cleavage takes place about 30 hours after fertilization. It is meridional, coinciding with the animal-vegetal pole axis. It produces two blastomeres, one slightly larger than the other. The two blastomeres remain adhered to each other. The second cleavage occurs within 60

hours after fertilization. It is at right angles to the plane of the first, and divides each blastomere into two by forming a mitotic spindle in each. The larger blastomere divides a little sooner than the smaller one so that there is a transitory “3-cell” stage before the characteristic “4-cell” stage of the embryo is reached. Third cleavage takes place about 72 hours after fertilization. Subsequent cleavage divisions follow one after another in an orderly manner, but in a less precise orientation. Cleavage produces a solid ball of small blastomeres.

1. Formation of morula : After 4^th cleavage solid ball consist of 16 to 32 cells are formed which looks as a little mulberry called morula. Due to holoblastic and unequal cleavage, two types of blastomere are formed.

There is an outer layer of smaller (micromere) transparent cells around on inner mass of larger cells (macromere). The morula reaches the uterus about 4 to 6 days after fertilization. It is still surrounded by the zona pellucida, that prevents its sticking to the uterine wall.

2. Formation of blastula (blastocyst) : It involves the dynamic rearrangement of blastomere. The outer layer of cell becomes that and form trophoblast or trophoectoderm which draws the nutritive material secreted by the uterine endometrial glands. The fluids absorbed by the trophoblast collects in a new central cavity called blastocoel or segmentation cavity or blastocystic vesicle.

As the amount of nutritive fluid increases in blastocoel, morula enlarges and takes the form of a cyst and is now called blastocyst or blastodermic vesicle. The cells of trophoblast do not participate in the formation of embryo proper. These cells form only protective and nutritive extra-embryonic membranes which later form foetal part of placenta e.g. chorion for placenta formation, amnion for protection from injury and dessication.

Inner cell mass of macromeres forms a knob at one side of trophoblast and forms an embryonal knob and is primarily determined to form the body of developing embryo so is called precursor of the embryo. The side of blastocyst to which embryonal knob is attached, is called abembryonic pole. Zona pellucida disappears at the time of blastocyst formation. The trophoblast cells in contact with the embryonal knob are known as cells of Rauber.

(viii)Types of blastulae :

1. Coeloblastula : A hollow blastula in which blastocoel is surrounded by either single layered (e.g. echinoderms, amphioxus) or many layered blastoderm (e.g. frog).
2. Stereoblastula : Solid blastula with no blastocoel e.g. in coelentrates annelids and molluscs.
3. Discoblastula : The blastula is as a multilayered flat disc at the animal pole lying on the top of well developed yolk. It is found in reptiles, birds, prototherians and fishes.
4. Blastocyst : In this, the blastula is as a cyst with 2 types of cells : an outer epithelium – like layer of trophoblast or nutritive cells; and an inner mass of formative cells collectively called embryonal knob.
5. Superficial blastula or periblastula : In this, the blastocoel is filled with yolk and is surrounded by a peripheral layer of cells. It is found in insects.

(ix)Significance of cleavage :

1. Cleavage restores the cell size and nucleo-cytoplasmic ratio characteristic of the species. It does not result in growth, though it increases cell number. During cleavage, cellular activity is till mainly controlled by the organelles and molecules received from the secondary oocyte’s cytoplasm, but some of the developing organism’s gene become active.
2. Cleavage beside producing a large number of cells by rapid divisions also segregates different substance present in the cytoplasm into different cells. These substances determine how the various cells develop later.

Important Tips

• Fate map : Diagram showing presumptive or prospective areas on the surface of blastula. It is done by using certain vital stains like neutral red, nile blue sulphate, bismarck brown, etc. It was first prepared by W. Vogt (1929).
• Zona pellucida disintegrates just after completion of cleavage.
• Cells of corona radiata disperse just before implantation.

  Implantation.                                                                                                                           

1. Definition : The process of attachment of the blastocyst on the endometrium of the uterus is called implantation.
2. Period : Though the implantation may occur at any period between 6^th and 10^th day after the fertilization but generally it occurs on seventh day after fertilization.
3. Mechanism ; First of all, the blastocyst is held closely against the uterine endometrial epithelium. The uterine capillaries and uterine wall in the immediate vicinity of the embryo become more permeable and a local stromal oedema is developed. Soon the endometrium around the embryo shows the first sign of a decidual cell reaction (DCR) which involves :

1. The epithelium becomes disrupted and the loosely packed fibroblast-like cells of the stroma are transformed into large rounded glycogen-filled cells.
2. The area of contact becomes more vascular.
3. The decidual cells form an “implantation chamber” around the embryo before the formation of a functional placenta.
4. The trophoblast is developed from the superficial layer of the morula stage. Later, the trophoblast is lined by mesoderm to form the chorion which contributes to the placenta formation.
5. Trophoblast of the chorion penetrates the uterine epithelium by both cytolytic and mechanical activity. The phagocytic activity of the trophoblastic cells through the decidual cells continues till it establishes intimate connection with the uterine blood vessels. The process of implantation is aided by proteolytic enzymes produced by the trophoblast. After implantation, endometrium undergoes many changes and forms decidua. It is differentiates into three parts such as : Decidua basalis present between the embryo and uterine myometrium, Decidua capsularis lies between the embryo and lumen of the uterus and Decidua parietalis is formed by the remaining part of decidua.

The pattern of implantation of the blastocyst varies in different species, which are as follows

1. Interstitial implantation : The blastocyst get burried into the endometrium e.g. human female, hedgehog, guinea pig, some bats and ape.
2. Central implantation : The blastocyst remain the uterine cavity e.g. rabbit, cow, dog and monkey.
3. Eccentric implantation : The blastocyst comes to lie in a uterine recess e.g. rats, mice.

(iv)Hormonal control of implantation

1. Role of estrogens : These are a group of steroid hormones mainly secreted by follicular epithelial cells of Graafian follicle though these are also produced by adrenal cortex and placenta. These include b-estradiol, esterone, estriol etc. Out of which most important estrogen is b-estradiol. Secretion of estrogens is stimulated by

FSH of anterior lobe of pituitary glands. These stimulate the uterine endometrial epithelium to enlarge, become more vascular and more glandular. The uterine glands become tortuous and cork-screw shaped. So the endometrium prepares itself for implantation. This stimulation by the estrogens on the uterus generally occurs on the 4^th day of pregnancy.

Meridional cleavage

1 Blasto

2                      3                          4                       5

Zygote

6

mere Cleavage begins 30 hr. after fertilisation

60 hr after fertilisation

Blastulation

Zona pellucida Inner cell mass Blastocoel

Trophoectoderm

7                            8

Morula

(=Trophoblast)

9

Hatching

Trophoectoderm (=precursor of

(32 cells) 4 days after

fertilisation

Inner cell mass Blastocoel

Early blastula

Syncytial trophoblast

Invading

extra embryonic

membranes)

10

Blastula hatches from

endometrium

11

zona pellucida                             Blastocyte during implantation

Endoderm

12

13

Differentiation of foetal membranes

Amnion

Amniotic cavity Epiblast

Yolk sac

Zygote

Cleavage

Fertilisation

Morula Blastula

Ovulation