Chapter 17 Human Reproduction Part 2 by TEACHING CARE online tuition and coaching classes

Chapter 17 Human Reproduction Part 2 by TEACHING CARE online tuition and coaching classes




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.

  • 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
  • Post natal or post embryonic period : It is the period of development which extend from hatching or birth to 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.

  • 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

  • Spermatogenesis : Formation of (b) Oogenesis : Formation of ova


Note :
  • (See detail in module-2 Chapter-Reproduction)
  • 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
  • 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
  • Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called
  • 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 It is with 3 primary layers.
  • Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and




Important Tips

Historical background of Embryonic Development :

  • George Newport : Observed fertilization of frog’s
  • Oscar Hertwig : Described the fusion of sperm and egg nuclei in sea
  • Prevost and Dumas : Reported cleavage of frog’s
  • Swammerdam : Observed the first cleavage of frog in 1738.
  • Spallanzani : Detailed process of cleavage of frog’s
  • Von Bear : Proposed recapitulation
  • Ernst Haeckel : Modified recapitulation theory to “Biogenetic law”. It states “Ontogeny repeats ”
  • 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 g. most of non-chordates, fishes, amphibians and reptiles; all birds and prototherians.
  • Ovoviviparity : Fetilization always 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 Placenta is formed and female gives birth to young one e.g. most of mammals.



  • 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
  • Site of fertilization : Fertilization in human female is internal as in other It takes place usually in the ampulla of the fallopian tube.

(iii) Steps of fertilization

  • 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

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 14th 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.





  • 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

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.

  • Cortical reaction : Immediately after the entry of a sperm into the egg, the later shows a cortical reaction to check the entry of more 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 :

  • The egg surface produces fertilization

Corona radiata





Zona pellucida


Dispersal of corona cells


  • The vitelline membrane is lifted and is converted into fertilization
  • The cortical granule
  • The cytoplasm exhibits
  • The permeability of plasma membrane
  • The coenzyme NAD is
  • The rate of protein synthesis
  • Mitosis is initiated.
  • The breakdown of polysaccharide

Zona pellucida Oolemma




Amphimixis            Zygote


  • The enzyme dehydrogenase increases.

Fig. Steps of fertilization




  • 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 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

  • External fertilization : In this, the gamete fuse outside the female body and is found in most of bony fishes (g. Labeo), amphibians (e.g. frog), all echinoderms (e.g. starfish) and lower chordates (e.g. Herdmania).
  • 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).
  • Self fertilization (Endogamy) : In this, two fusing gametes are derived from the same parent (uniparental) g. Taenia, Fasciola (sheep, liver fluke).
  • 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 g. Pheretima (earthworm-due to protandry), Scypha (Sycon-due to protogyny) Fasciola and Taenia (have both self and cross fertilization).
  • Monospermic fertilization : When only one sperm enters and fuses with ovum. It is found in most of
  • 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

(v) Significance of fertilization

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




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

Important Tips

  • Termones : Chemical released by algae in water for attraction of
  • Pheromones : Chemical released by insects in air and generally acts as sex attractants g. in gypsy moth.
  • Gamones : Chemical released by the human gametes for their
  • Zygote is called the first cell of next
  • Isogamy : When two fusing gametes are morphologically and physiologically similar g. monocystis.
  • Anisogamy : When two fusing gametes are morphologically and physiologically different g. frog, human beings etc.
  • Twins : When 2 or more babies are born in multiple births then these are called 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
  • Free martins : A sexually under-developed female calf joined with a
  • Polyspermy : Penetration of many sperms into an ovum simultaneously. Only one of the spermatozoa will be successful in uniting with female
  • Polygyny : When two female pronuclei unite with a male
  • Polyandry : Conjugation of two or more male pronuclei with a female
  • Gynogenesis : Activation of egg by sperm, but there is no fusion of its
  • Androgenesis : Non-participation of female pronucleus in
  • Cone of reception (Fertilization Cone) : A conical outgrowth given by egg of frog to receive the sperm. Not found in human
  • Fertilizin is a glycoproteinous or mucopolysaccharide molecule, while antifertilizin is a proteinaceous substance of acidic amino acids on the surface of head of
  • Fertilizin-Antifertilizin reaction was proposed by 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
  • The slow block to polyspermy develops, in response to the formation of the fertilization membrane and within a minute after the fast
  • The motion of sperm is
  • Polyspermy is of common occurance in
  • Bindin is a protein in acrosome which ensure that the egg is being fertilized by a sperm of the same



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




  • 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




S.No. Characters Cleavage Normal mitosis
(1) Site of occurrence In zygote or parthenogenetic egg In most of somatic cells
(2) Interphase Of shorter period Of longer period
(3) Growth Does not occur Occurs during interphase
(4) Oxygen consumption High as is very rapid process Low as is slow process
(5) Size of daughter cells Decreases Remains same after growth
(6) DNA synthesis Faster Slower
(7) Nuclear-cytoplasmic ratio Increases Remain same
  • 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 They are as follows.
  • 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

Example : Ist and IInd cleavage furrow of frog and Ist cleavage furrow of chick.

  • 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

Example : IIIrd cleavage furrow of chick.

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

Example : Ist cleavage plane of eggs of higher mammals.

  • 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

Example : IIIrd 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

  • 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 They are as follows





  • 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

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)



  • 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)


  • Spiral cleavage : The spiral cleavage is diagonal to the polar 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


  • 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

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

Presumptive                      Slaty gray cytoplasm                notochord

Yellow cytoplasm







Mesenchyme cells

Fig : Bilateral cleavage


Presumptive plate






Muscle cells


  • Cleavage on the basis of potency : According to potentialities of early blastomeres, cleavage may be of following
  • Determinate cleavage or mosaic cleavage : In determinate cleavage, each early blastomere is destined to become a particular portion of

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





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

Example : All chordates, echinoderms and arthropods.

  • Types of cleavage : The amount of yolk (Lecithality) also determines the type of Which are as follows
  • 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 Accordingly they are of two types
    • Equal holoblastic cleavage : If the blastomeres are approximately equal, it is called equal holoblastic

Examples : Echinoderms, amphioxus and placental mammals.

  • 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


Example : Fish and amphibians.







blastomere blastomere

Second furrow


Zonal pellucida Polar bodies


Zona pellucida Blastomeres


Animal hemisphere

First furrow





Albumen                      Albumen

Zygote                   2-cell stage                          Morula

Fig : Holoblastic equal cleavage

Grey crescent

Vegetal hemisphere

Fig : Holoblastic unequal cleavage


  • 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
    • 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

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

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

Example : Centrolecithal eggs of arthropods.

B                     A





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


  • Cleavage in human zygote : Cleavage in the human zygote occurs during its passage through the fallopian tube to the uterus as in other 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.

  • Formation of morula : After 4th 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

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.

  • 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

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 :

  • Coeloblastula : A hollow blastula in which blastocoel is surrounded by either single layered (g. echinoderms, amphioxus) or many layered blastoderm (e.g. frog).
  • Stereoblastula : Solid blastula with no blastocoel g. in coelentrates annelids and molluscs.
  • Discoblastula : The blastula is as a multilayered flat disc at the animal pole lying on the top of well developed It is found in reptiles, birds, prototherians and fishes.
  • 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
  • Superficial blastula or periblastula : In this, the blastocoel is filled with yolk and is surrounded by a peripheral layer of It is found in insects.

(ix) Significance of cleavage :

  • Cleavage restores the cell size and nucleo-cytoplasmic ratio characteristic of the 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.
  • Cleavage beside producing a large number of cells by rapid divisions also segregates different substance present in the cytoplasm into different These substances determine how the various cells develop later.




Important Tips

  • Fate map : Diagram showing presumptive or prospective areas on the surface of 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
  • Cells of corona radiata disperse just before



  • Definition : The process of attachment of the blastocyst on the endometrium of the uterus is called
  • Period : Though the implantation may occur at any period between 6th and 10th day after the fertilization but generally it occurs on seventh day after
  • Mechanism ; First of all, the blastocyst is held closely against the uterine endometrial 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 :
  • The epithelium becomes disrupted and the loosely packed fibroblast-like cells of the stroma are transformed into large rounded glycogen-filled
  • The area of contact becomes more
  • The decidual cells form an “implantation chamber” around the embryo before the formation of a functional
  • 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
  • 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 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

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

(iv) Hormonal control of implantation

  • 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 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 4th day of pregnancy.


Meridional cleavage





1 Blasto

2                      3                          4                       5










mere Cleavage begins 30 hr. after fertilisation

60 hr after fertilisation




Zona pellucida Inner cell mass Blastocoel



7                            8









Trophoectoderm (=precursor of

(32 cells) 4 days after


Inner cell mass Blastocoel

Early blastula


Syncytial trophoblast




extra embryonic



Blastula hatches from




zona pellucida                             Blastocyte during implantation















Differentiation of foetal membranes


Amniotic cavity Epiblast

Yolk sac











Morula Blastula


Ovulation                                                                             Implantation




Fig : Cleavage and stages of implantation of blastocyte differentiation of foetal membranes can also be seen





  • Progesterone : It is also a steriod hormone secreted by yellow-coloured endocrine gland, called corpus luteum, formed from empty Graafian folicle during the Small amount of progesterone is also secreted by adrenal cortex and placenta. Secretion of progesterone is stimulated by LH of anterior lobe of pituitary gland.

Progesterone acts on only those uterine cells which have been earlier stimulated by estrogens. Progesterone further stimulates the proliferation of endometrium of uterus and prepared, placenta formation and normal development of the foetus in the uterus.


  • Definition : Gastrulation is a dynamic process involving critical changes in the embryo such as differentiation of cells, establishment of the three primary germ layers and transformation of the single walled blastula into a double walled gastrula.
  • Types of gastrular movement or morphogenetic movement : The movements of cells during gastrulation is called formative or morphogenetic movements. Following types of gastular movements are found in different animals
  • Epiboly : It involves the morphogenetic movement of prospective ectodermal (micromeres) blastomeres antero-posteriorly to envelop the presumptive endodermal and mesodermal blastomeres. It is found in telolecithal egg of
  • Emboly : It involves inward movement of prospective endodermal and chorda-mesodermal blastomeres from the surface of
  • Invagination : It involves insinking of endodermal cells in the blastocoel to form It is found in amphioxus.
  • Involution : It involves the rolling in of the chorda-mesodermal blastomeres inside the ectodermal cells over the lips of It is also found in the gastrulation of frog.
  • Convergence : It involves migration of blastomeres from the outer surface towards the blastoporal
  • Ingression or polyinvagination : In this, individual blastomeres migrate into the blastocoel either from only vegetal pole (called unipolar ingression g. Obelia; or form all sides called multipolar ingression e.g. Hydra) to form a solid gastrula called stereogastrula.


  • Delamination : It involves splitting off the blastoderm into two layers by the appearance of grooves resulting the formation of hypoblast. It is found in
  • Formation of layers by gastrulation : Gastrulation includes the formation of following structures
  • Formation of endoderm : The blastodermic vesicle enlarges and cells present on the lower surface of the embryonal knob detach by delamination from the embryonal The detached cells become flat,

Epiblast Embryonal knob

Beginning of

endoderm Blastocoel


  • (B)

Aminotic cavity

Aminogenic cells Epiblast Embryonic endoderm


Primary yolk sac


divide increase in number and form the endoderm inside the trophoblast of

the blastodermic vesicle. The embryo at this stage is tubular and encloses a






hollow tube (called primitive gut or archenteron) lined by endoderm. The

Fig : Formation of endoderm and ectoderm





part of endoderm located under the embryonal knob is called embryonic endoderm which later forms embryonic gut, while the remaining part of endoderm along with trophoblast forms the yolk sac.

  • Formation of embryonic disc and mesoderm : Meanwhile, the blastocyst continues to grow due to absorption of more and more uterine milk. The embryonal knob stretches and cells of Rauber start breaking off and So the cells of embryonal knob from a regular layer called embryonic disc which becomes continuous with the trophoblast. Embryonic disc is differentiated into cephalic, embryonic and caudal regions. Formation of embryonic mesoderm starts at the caudal region of the embryonic disc where cells undergo rapid proliferation and form a localized thickening of the embryonic disc and form the mesodermal layer between ectoderm and



Embryonic disc












Aminiotic Ectoderm

Endoderm Secondary yolk

sac Primary yolk Endoderm


embryonic     Extra embryonic



Embryonic disc








Fig : Formation of extraembryonic mesoderm and coelom

(c) Formation of ectoderm : The remaining cells of blastodisc become columnar and form ectoderm.

  • Fate of germ layers : Each of the three germ layers gives rise to definite tissues, organs and systems of the Their fate in embryo and adult has been listed below

Fate of germ layer


Ectoderm Mesoderm Endoderm
Epidermis and skin derivatives Dermis Gut
Cutaneous gland Muscular tissue Glands of stomach and intestine
Nervous system (Brain + spinal cord) Connective tissue Tongue
Motor and optic nerve Endoskeleton Lung, trachea and bronchi
Eye (Retina, lens and cornea) Vascular system (heart and blood vessel) Urinary bladder
Conjuctiva, ciliary and iridial muscle Kidney Primordial germ cells
Nasal epithelium Gonads (Reproductive system) Gills
Internal ear (membranous labyrinth) Urinary and genital ducts Liver
Lateral line sense organ Coelom and coelomic epithelium Pancreas
Stomodaeum (mouth) Choroid and sclerotic coat of eye Thyroid gland
Salivary gland Adrenal cortex Parathyroid gland
Enamel of teeth Spleen Thymus
Proctodaeum Notochord Middle ear
Pituitary gland Parietal and visceral peritoneum Eustachian tube
Pineal body   Mesoderm (Mid gut)
Adrenal medulla   Lining of vagina and urethra
Hypothalamus   Prostate gland




  • Significance of gastrulation
  • Three primary germ, layers are
  • It marks the beginning of morphogenesis and
  • Metabolic activities of the cells are increased due to great morphogenetic activities of the


Important Tips

  • First embryonic membrane to be formed is
  • Embryonal knob is called precursor of the embryo, while trophoblast forms protective and nutritive extra-embryonic membranes.
  • Cells of Rauber : Those cells of trophoblast which are in contact with embryonal
  • Yolk sac is also found in some anamniotes like certain cartilage fishes (e.g. scoliodon), bony fishes and few amphibians (e.g. Necturus) having polylecithal
  • Protostomous : When blastopore forms the mouth in development and is found in non-chordates except echinoderms, hemichordates and
  • Deuterostomous : When blastopore forms the anus in Mouth is developed later than anus e.g. echinoderms hemichordates and chordates.

 Neurulation and organogenesis.

Post gastrulation involves two main process. Neurulation is process of laying the neural plate to form the nervous system. The establishment of the germ layers initiates the final phase of embryonic development, i.e., organogenesis. The latter involves differentiation and specialization of groups of cells in the individual germ layers. The cells of such groups change their form and give rise to morphologically recognizable tissues and organs of the new individual. The groups of differentiated cells separate from their germ layers in an orderly manner and with unique precision. Separation of the differentiated cell groups may occur by folding off from the germ layer or by migration of cells individually and reaggregation at a new place. In this manner, the primordial cells of the germ layers gradually and accurately give rise to the tissues and organs of the offspring.

By four weeks after fertilization, the embryo has a simple heart, limb buds and eye rudiments. It also has a tail and pharyngeal pouches, the vestiges of its early vertebrate ancestors that disappear later in development. After the second month, the embryo is recognizable as a primate. From this stage on, the embryo is often called foetus.

Some important events in the human development


Time from fertilization Stage/organs Event
24 hours Cleavage Embryo is at two-cell stage
3 days Morula The morula reaches uterus
7 days Blastocyst Implantaion of blastocyst begins
2.5 weeks Notochord Notochord formed, differentiation of tissues that will give rise to heart, blood cells formed in yolk sac and chorion.
3.5 weeks Organ system Neural tube formed, primordial eye and ear vesicle, pharyngeal pouches formed, liver bud differentiates, respiratory system and thyroid gland begin to develop, heart tube bends and begins to beat, blood vessels are formed.
4 weeks Limb buds Development and appearance of limb buds, brain forms three primary vesicles.
2 months Muscles and gonads Muscles differentiate, embryo capable of movement, gonads distinguishable as

testes or ovaries, ossification of bones begins, cerebral cortex is differentiated,





    blood vessels assume final position.
3 months Sex differentiation By external examination sex can be determined, notochord degenerates, lymph glands develop.
4 months Face Face begins to look human, eye, ear and nose look ‘normal’, cerebral lobes differentiate.
6-9 months (third trimester) Lanugo (hairs) body


Lanugo appear but are shed later, tremendous growth of body occurs, neurons become myelinated
266 days Birth (parturition) Baby is born, neonate arrives in outer world.


 Extra embryonic membrane.

An aquatic embryo is surrounded by water, which protects the embryo, keep it moist, removes wastes and permits gas exchange. In land vertebrate (reptiles, birds and mammals), these functions are taken over by the extraembryonic membranes. These membranes are formed outside the embryo from the trophoblast only in amniotes and perform specific function. Some of these membranes take part in the formation of placenta in mammals.

  • Yolk sac : It is formed below the embryo. It contains fluid, not yolk. The yolk sac is a vestigeal organ inherited from the oviparous reptilian ancestors. Yolk sac encloses by outer mesoderm and inner endodermal

Function : It is mainly digestive in function. It also absorbs the dissolved yolk and passes it to developing embryo in reptiles, birds and prototherian. In human beings, it is vestigial. In human embryo it act as the site of blood cell formation until about the 6th week, when the liver takes over this role.

  • Amnion : It is formed above the It consist of outer mesoderm and inner ectoderm. The amnion and the fluid filled amniotic cavity it encloses, enlarge and nearly surround the embryo. The embryo is suspended in the amniotic cavity by an umbilical cord. The latter is formed of the stalks of the yolk sac and allantois. The main blood vessel from the placenta reach the foetus through the umbilical cord. Amniotic fluid secreted by both embryo and amnion. The cells of amniotic fluid are the basis of parental test called amniocentesis, for the sex of the foetus and for checking chromosomal defects in it.


  • The amniotic fluid cushions the
  • It protecting embryo against bumps and bacterial
  • It maintains a constant temperature and
  • It protects the embryo from jerk, injury and shocks.
  • It prevents desiccation of the embryo.
  • Allantois : It is a fold of splanchnopleur developed from the hind gut of the embryo. It consist of outer mesoderm and inner endoderm. It is well developed in amniotes with polylecithal egg (g. reptiles, birds and prototherians) and stores the nitrogenous waste of the embryo so act as extra embryonic kidney. In most of eutherians, it combines with chorion to form allantochorion placenta. But in man it remains small or reduced and





does not reach the chorion. However, it forms umbilical arteries and veins which grow up to the chorion to vascularise it.


  • The cavity of the allantois serves as a urinary bladder. It stores the protein breakdown product in the form of water-insoluble crystals of uric acid and inside the egg upto the time of hatching. But with the acquisition of viviparity in the marsupials and the placental mammals, the original function of the allantois as a urinary bladder becomes altogether
  • The vascular “chorioallantoic membrane” lies in a close proximity to the inner surface of the porous It acts as an extraembryonic lung by supplying the embryo with oxygen.
  • Together with the chorion, the allantois also surrounds the albumen to form the albumen sac and thus assists in the absorption of nutritionally rich albumen.
  • In mammals, allantois supply oxygen and nutrient to the
  • Chorion : It is outermost fold of somatopleur (outer ectoderm and somatic mesoderm) and surrounds the In reptiles birds and prototherians, allantochorion act as extra embryonic lungs help in exchange of gases. But in primates including human beings, only chorion forms the placenta (chorionic placenta).

Function : It protects the embryo and forms placenta for metabolic exchange between the foetus and the



Ectoderm Endoderm




Ectoderm Mesoderm

Amniotic cavity


Embryo Embryonic gut








Endoderm Mesoderm

Extra-embryonic coelom


Mesoderm Endoderm

Placental chorionic villi

Yolk sac


Fig : Foetal membranes and placenta (early stage)

Types and functions of extra embryonic membrane


S.No. Name of membrane Characteristics and functions Remarks
(1) Yolk sac (1)        Formed by inner endoderm and outer mesoderm (= splanchnopleura)

(2)          Digestive function (= extra embryonic duct)

(3)          Absorbs dissolved yolk and supplicate it to developing embryo.

Vestigeal in humans.


Well developed in reptiles, bird and prototherians.

(2) Amnion (1)     Formed by inner ectoderm and outer mesoderm (somatopleur) above the embryo.

(2)    Between the embryo and amnion there is a cavity called amniotic cavity filled with amniotic fluid secreted by amnion and embryo.

(3)     Amniotic fluid act as shock absorber and prevent desiccation of embryo.

(4)     Ex-foliated embryonic (= foetal) cells are used for (a) pre-natal sex determination (b) congenital defects (c) inborn metabolic



Well developed in all amniotes






    disorders. This technique is called amniocentesis.  
(3) Allantois Develops as fold of splanchnopleur; developed from gut of embryo. (1)      In placentals, it combines with chorion to form allanto- chorion placenta. (reduced in human)

(2)        Acts as extra-embryonic kidney in reptiles, birds and prototherians.

(4) Chorion Outermost fold of somatopleur and surrounds the embryo (1)     Takes part in forming the true chorio allantoic placenta

(2)        Acts as extra-embryonic lung in reptiles, birds and prototherians.



  • Definition : Placenta is defined as a temporary intimate mechanical and physiological connection between foetal and maternal tissues for the nutrition, respiration and excretion of the
  • Structure : Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine The latter becomes very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane and the uterine wall is known as the placenta. The placenta has two parts : the part contributed by the foetus, i.e.,


chorionic villi, is called the foetal placenta; and the part shared by the mother, i.e., part of uterine wall, is termed the maternal placenta. The chorionic villi receive blood from the

embryo by umbilical artery








Umbilical artery

Umbilical Umbilical


Chorion                                                                               Endometrium







and return it by umbilical vein. These blood vessels are derived from the allantois and run between the foetus and the uterine wall in the tough umbilical cord covered with cells derived from the amnion and chorion. The choroinic villi come to lie in uterine lacunae that receive blood












Foetal artery Foetal vein















Maternal blood space

Lacuna Chorionic villus

















Uterine arteriole



Uterine venule


from the uterine arteriole

and return  it by uterine

Fig : Diagram showing relationship between foetal and maternal blood




venule. The cells forming the wall of chorionic villi bear microvilli which increase their surface area for quick and adequate exchange of materials by diffusion, active transport and pinocytosis.

The placenta is fully formed by the end of the third month and it lasts throughout pregnancy. When complete, it is a reddish – brown disc. In the placenta, the foetal blood comes very close to the maternal blood, and this permits the exchange of materials between the two. Food (glucose, amino acids, simple proteins, lipids), water, mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and foetal metabolic wastes, such as carbon dioxide and urea, also water and hormones, pass into the maternal blood. The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The continuous uptake of oxygen by foetal blood is ensured by the difference in affinity for oxygen between foetal and maternal haemoglobin.

The maternal and foetal blood are not in direct contact in the placenta, because (i) the two may be incompatible; (ii) the pressure of maternal blood is far too high for the foetal blood vessels; and (iii) there must be a check on the passage of harmful materials (blood proteins, germs) into the foetal blood.

(iii) Functions

  • Placenta helps in the nutrition of the embryo as the nutrients like amino acids, monosugars, vitamins, etc. pass from the maternal blood into foetal blood through
  • It also helps in respiration of the embryo as O2 of the maternal blood and CO2 of the foetal blood diffuse through placenta into the foetal blood and maternal blood
  • It also helps excretion of the embryo as nitrogenous wastes of foetal blood like urea pass into maternal blood through
  • It also acts as an endocrine gland as it secretes certain hormones like estrogens, progesterone and human chorionic gonadorophin (HCG). HCG maintains the corpus luteum for the continued secretion of progesterone to maintain the pregnancy. At the end of gestation period, it also secretes relaxin which helps in softening of pubic symphysis and child birth. It also secrete small amounts of chorionic thyrotrophin, chorionic corticotropin and chorionic
  • Antibodies against diphtheria, smallpox, scarlet fever, measles, pass from maternal blood to foetal blood through placenta and provide passive immunity.
  • It stores the glycogen till the formation of
  • Though the placenta acts as an effective barrier for certain toxic chemicals like histamine but certain germs like AIDS virus, syphilis bacteria, viruses of German measles, etc, intoxicants like nicotine of cigarette smoke; and addictive drugs like heroin and cocaine can pass through the placenta and cause the developmental

(iv) Placenta and disease

  • Viral and bacterial infections of placenta are known as
  • If the mother suffer from certain diseases like syphilis, smallpox, chickenpox, AIDS and measles; their pathogen enter into the foetus through
  • Many drugs used medicinally may penetrate the placental





  • Drugs such as thalidomide taken as a sedative by woman in early pregnancy (25 to 44 days) cause extensive deficiencies in the development of limbs, the alimentary canal (non perforation of the anus) and the heart in
  • Nicotine from cigarette smoke crosses the placenta and stunts foetal
  • Addictive drugs such as heroin and cocaine reach the foetus, causing addiction to the new

(v) Classification of placenta

  • According to the foetal membrane involved in the formation of placenta.
  • Yolk sac placenta : In metatheria or marsupials, such as kangaroo (macropus) and opossum (Didelphys), placenta is derived from yolk sac and chorion. In metatheria, yolk sac placenta is only weakly developed so that embryonic nutrition and growth remain limited and the young is born very small and To compensate the deficiency of intrauterine development, it is transferred to the abdominal pouch or marsupium and fed on milk until fully formed. In higher mammals (Eutheria) a yolk sac placenta is usually not found but in some mammals (Hedgehogs and rabbit) it may be temporarily develop in early stages.
  • Allantoic placenta : In the majority of Eutherian, the chief organ of embryonic nutrition is the allantoic placenta consist of allantois and chorion and also called allantochorionic placenta. Outside Eutharia, a primitive allantoic placenta occurs only in perameles (bandicoot) which is a
  • Chorionic placenta : It occurs in primates (man and apes) and is formed only by chorion. Allantois remains small, burrows into body stalk (umbilical cord) and does not reach However, its mesoderm and blood vessels grow upto chorion whose villi enter the uterine crypts forming chorionic placenta.
  • According to the intimacy between the foetal and maternal part. Histologically there are six barriers are found in placenta which are as
    • Endothelium of foetal blood vessels


  • Chorionic connective tissue
  • Chorionic epithelium
  • Uterine epithelium
  • Uterine connective tissue
  • Endothelium of maternal blood vessel







On the presence or absence of above barriers histologically placenta is divided into following types

  • Epithelio-chorial : Most primitive and simplest type with all six placental Examples : Odd hoofed mammals such as horse, ass, pig and lemurs.
  • Syndesmo-chorial : Uterine epithelium absent, with five placental barriers. Examples : Even hoofed mammals such as cow, sheep, goat, camel
  • Endothelio-chorial : Uterine epithelium and uterine connective tissues are absent, with four placental





Examples : Carnivores (dog, cat, lion, tiger etc.), Tree shrew and mole.

  • Haemo-chorial : Uterine epithelium, uterine connective tissue and endothelium of maternal blood vessel absent, with 3 foetal layers.

Examples : Primates (man, apes and monkey).

  • Haemo-endothelial : Foetal capillaries indirect contact with maternal blood, only one placental Examples : Rat, guinea pig and rabbit.











Foetal side

(B)                          (C)                           (D)                            (E)


Foetal blood 1


Maternal blood




Maternal side


Fig : Histological types of placenta


  • Epithelio-chorial, (B) Syndesmo-chorial, (C) Endothelio-chorial, (D) Haemo-chorial, (E) Haemo-endothelial,

(1) Endothelium of foetal blood vessel, (2) Chorionic connective tissue (3) Chorionic epithelium, (4) Uterine epithelium,

(5) Uterine connective tissue (6) Endothelium of maternal blood vessel


  • According to shape and distribution of villi : Depending on the shape of placenta, manner of distribution of villi, degree of connection between foetal and maternal tissues and behaviour of placenta at the time of birth, the following types and subtypes of allantoic placenta can be


  • Non deciduous placenta : In most mammals villi are simple, unbranched and merely opposed without intimate contact between foetus and uterine At the time of birth or parturition, villi

are easily withdrawn from maternal crypts without

Arealae of villi




Chorionic folds

Cotyledon or areala of villi


causing any tissue damage. Thus no part of uterine tissue comes out and no bleeding occurs. Non deciduous or non-deciduate placenta has following subtypes according to the manner of distribution of villi.

  • Diffuse (B) Cotyledonary Compound villi


  • Diffuse : Villi remain scattered all over the surface of e.g. pig, horse, lemur.
  • Cotyledonary : Villi are arranged in separate tufts or patches called e.g. goat, sheep, cow, deer.
  • Incomplete zonary
  • Zonary (E) Discoidal Primary placenta

with compound villi


Secondary placenta

  • Metadiscoidal (podiscoidal)


  • Intermediate : Villi are arranged in cotyledons as well as e.g. camel, giraffe.

Fig : Types of placentae according to the distribution of villi




  • Deciduous placenta : Villi are complicated, branched and intimately connected. At birth, a variable amount of maternal tissue is pulled out with the shedding of blood. Deciduous or deciduate placenta is also differentiated in the following subtypes
  • Zonary : Villi form an incomplete (g. racoon) or complete girdle encircling the blastocyst. e.g. cat, dog,


  • Discoidal : Villi are restricted to a circular disc or plate on the dorsal surface of e.g.

insectivores, bats, rodents (rat, mouse), rabbit, bear.

  • Metadiscoidal : Villi are at first scattered but later become restricted to one or two discs. It is monodiscoidal in man and bidiscoidal in monkeys and
  • Contra-deciduous : Foetal villi and uterine crypts are so intimately connected that even most of foetal placenta is left behind at birth to be broken and absorbed by maternal leucocytes g. bandicoot (perameles), mole (Talpa).

Important Tips

  • Yolk sac is also found in some anamniotes like certain cartilage fishes (e.g. Scoliodon), bony fishes and a few amphibians (e.g. Necturus) having polylecithal
  • Amnion, chorion and allantois are formed only in amniotes embryo as their development occurs on land either inside egg or in the uterus of These are not formed in anamniotes as their development occurs in water so there is no problem of dessication, supply of oxygen and removal of oxygen.
  • Splanchnopleur : A fold formed of endoderm and splanchnic
  • Somatopleur : A fold formed of ectoderm and somatic
  • Amniocentesis : Prenatal diagnostic technique in which amniotic fluid is withdrawn to know sex of developing foetus congential chromosome defects and inborn errors of
  • Human placenta is fully formed in about 10 weeks and lasts throughout
  • Presence of HCG in the urine sample of a female confirm the


 Gestation period and parturition.

  • Gestation period : Gestation period is the duration between fertilization and parturition.

Gestation period


S.No. Animal Days
(1) Mouse (Minimum) 19-20
(2) Rat 20-22
(3) Rabbit 28-32
(4) Cat 52-65
(5) Dog 60-65
(6) Pig 112-120
(7) Goat 145-155






(8) Man 270-290
(9) Cow 275-290
(10) Horse 330-345
(11) Elephant (Maximum) 607-641


  • Parturition : It is the expelling of the fully formed young from the mother’s uterus after the gestation period (about 280 days in human female).
  • Mechanism : A developing foetus secretes hormones from its adrenal These hormones diffuse into the maternal blood and accumulate to


stimulate the release of oxytocin (birth hormone) from the mother’s posterior pituitary. Oxytocin causes the forceful contraction of smooth muscles of myometrium, called labour pains which pushes the young gradually out through the dilated cervix (caused by relaxin) and vagina, with the head foremost. It is aided by a reflex (whose centre lies in the lumbar region of spinal cord) and voluntary contraction of abdominal muscles. In the beginning, the labour pains occur once every half or quarter of an hour but soon become more frequent. The foetal membranes burst and amniotic fluid is released but foetal membranes remain behind. This expulsion stage lasts about 20 minutes to one hour. It is followed by placental stage























Amnion presses

against cervix


  1. Cervix is beginning to dilate in response to pressure of the head of the baby and contracts under the dual influence of oxytocin and Relaxin softens lower part of uterus and relaxes pubic symphsis.




Contraction of uterine myometrium

Dilation of cervix





  1. Passage of the body (head foremost) through the birth


of 10-45 minutes during which the umbilical cord, placenta and foetal membranes are expelled as decidua or after birth. It is

Fig : Two stages in parturition. The placental hormones play a critical role in

tandem with oxytocin


because after the child birth, the uterus reduces in size causing detachment of placenta. Umbilical cord is tied and then cut which finally shrinks into a depressed scar called umblicus or navel. Sometimes, the foetus fails to come out then the baby is delivered by a surgical procedure. Such a baby is called cesarean.

  • Control : Parturition is controlled by hormones :
    • Oxytocin : Causes powerful contractions of myometrium during
    • Relaxin : Causes widening of pelvis by relaxing the pubic symphysis of the pelvic

Important Tips

  • Teratogens are those physical, chemical and biological agents, which may cause malformations in the developing
  • Teratology : Branch of developmental biology which deals with the study of abnormal development during
  • Morning sickness : A desire for unusual food during




  • Gestosis : Any disorder of
  • Postpartum care : Care after
  • Lanugo : Most of the body of foetus is covered with downy hairs called lanugo which are generally shed before birth
  • Uterine milk : Nutritive endometrial
  • Miscarriage or Abortion : Loss of embryo due to breakdown of endometrium due to lowering of progesteron secretion from corpus
  • Nidiculous or Altricial young : Underdeveloped and helpless young born g. cats, dogs, rats, etc.
  • Phocomelia : Abnormal baby born with flipper-like limbs due to continued use of Thalidomise (sleeping pills) by the pregnant This drug passes through placental care.


 Development in frog.

(i) Breeding

  • Frog breeds in the rainy season, June to
  • Male frogs produce crocking sound (mating call) by their vocal
  • The sexual embrace of the male and female frogs is called amplexus (false copulation).

(ii) Ovulation

  • Ovulation is the release of eggs from ovary in the body
  • The eggs in the stage of secondary oocytes are released into the body cavity by rupture of ovary during

(iii) Spawning

  • Spawning is the act of laying of eggs by the female frog stimulated by the male during
  • Spawn is a cluster or mass of eggs laid by a
  • A spawn of Rana tigrina contains about 3000-4000
  • The diameter of frog’s egg varies from about 75 to 2.5 mm.
  • The egg is surrounded by a thin vitelline membrane and three layers of jelly coats made of
  • Gelatin protects the egg from predators and also acts as an insulator keeping the egg warm.

(iv) Fertilization

  • Fertilization in frog is external taking place in
  • The sperms are released on the egg mass before it reaches
  • When a sperm enters into the egg of frog, second meiotic division occurs.
  • A sperm enters into the ovum at some point in animal
  • A gray crescent appears in the equitorial zone geometrically opposite to the sperm entrance.
  • Gray crescent marks the dorsal side of the future
  • Sperm entrance point marks the anterior side of the future
  • The bilateral organization is established at the time of sperm
  • The region where sperm enter the egg cell is called ‘reception cone’.
  • Entry of sperm induces following changes in the ovum
  • Formation of fertilization membrane around the
  • Completion of second maturation division of egg
  • Activation of the egg for
  • Fusion of male and female pronuclei, e., amphimixis.




  • Formation of gray

(v) Structure of egg

  • Frog’s egg is mesolecithal (based on distribution of yolk).
  • Upper black of darkly pigmented part is animal hemisphere. Lower unpigmented or white part is vegetal hemisphere.
  • Cytoplasm is concentrated in animal pole. It is directed dorsally and pigmented animal pole is related with camouflage, to escape notice of
  • Vegetal hemisphere of frog’s egg contain yolk. It remains directed
  • The correct sequence in the development of frog is fertilization, cleavage, morula, blastula and gastrula.

(vi) Cleavage

  • Cleavage is a term used for the early cell divisions of the zygote upto the completion of blastula
  • First cleavage of frog is meridional passing through median longitudinal axis, holoblastic and
  • The first cleavage furrow appears at animal pole and results in two blastomeres, right and
  • The second cleavage is right angle to first one, again meridional results in four blastomeres, identical with respect to cytoplasm, pigment and yolk
  • Second cleavage is again holoblastic and
  • Gray crescent is present only in two blastomeres of future dorsal
  • All divisions from third cleavage are unequal holoblastic.
  • Holoblastic equal cleavage in frog ends after second cleavage division.
  • Third cleavage plane is horizontal, but above the level of equator (latitudinal).
  • Third cleavage results in the formation of embryo with eight blastomeres, four upper one are smaller and pigmented and four lower most are larger and yolk
  • Smaller cells are micromeres and larger ones are called macromeres. The micromeres contain no yolk and macromeres contain large amount of
  • Fourth cleavage involves two synchronous meridional divisions resulting in the formation of 16
  • Fifth cleavage involves two simultaneous latitudinal divisions resulting in the formation of 32 blastomeres.
  • The divisions after the fifth cleavage becomes very irregular and asynchronous. The micromeres divide faster than
  • After sixth or seventh cleavage division, the embryo looks like a mulberry-shaped ball of This called morula stage.
  • Morula is a solid ball of A cavity called blastocoel appears towards animal hemisphere. The blastula of frog is called coeloblastula.
  • Blastocoel is filled with an albuminous fluid secreted by surrounding
  • The two cells of frog’s egg formed by the first cleavage represent the right and left half of the
  • In frog, there is a regulative development. The cleavage is indeterminate. If one of the two blastomeres of frog is damaged, the development will be

(vii) Gastrulation

  • Gastrulaion is the process by which a blastula is converted into




  • Blastula is a hollow ball of cells. By the end of gastrulation, it is converted into a three-layered embryo made of ectoderm, mesoderm and endoderm often enclosing an archenteron.
  • Gastrulation includes three kinds of morphogenetic movements of cells namely
  • Epiboly of ectoderm
  • Invagination of endoderm
  • Involution of chordamesoderm
    • Epiboly is migration and spreading of micromeres over the embryo is known as epiboly.
    • Invagination of prospective endoderm cells occurs below equator, exactly below the midpoint of gray crescent of It results in the formation of a slit later giving rise to blastopore.
    • Involution is the insinking and movement of chordamesoderm cells towards anterior side along the roof of blastocoe
    • Gastrulation results in the formation of a new cavity, archenteron which opens outside through blastopore.
    • Archenteron is present the lumen of future
    • Blastopore occurs in gastrula and opens into
    • Blastopore will give rise to future anus in
    • If involution does not takes place (chordamesoderm cells evaginates instead of involution) no gastrulation takes place. The development will be
    • Ingression is the migration of individual vegetal cells to the interior of the
    • By the end of gastrulation, blastocoel will be reduced. A yolk plug of endodermal origin closes the
    • Posterior side of future tadpole is represented by the side of frog’s embryo bearing the yolk

(viii) Organizer

  • The dorsal lip of blastopore in the amphibian gastrula is called primary organizer.
  • The theory of organizer (inductor) in amphibian was introduced by Spemann in 1938. He was awarded Nobel prize for this

(ix) Neurulation

  • Neurulation takes place after During this stage a neural tube is formed.
  • The embryo lengthens along its anterior-posterior axis, neural plate (ectodermal) become thickened and raised above the general level as ridges called neural
  • Neural folds meet and fuse at the mid dorsal
  • Neurulation includes the formation of neural tube, notochord and
  • Formation of notochord is known as notogenesis.

(a)  Post neurular development

  • The development takes place inside egg membrane upto tail bud larval
  • Hatching occurs in 6th day of embryonic
  • During hatching, the young frog is called tadpole
  • Newly hatched tadpole larva remain attached to aquatic plants by its oral
  • After 24 hours of hatching, mouth and anus are
  • The larval body is elongated forming head, trunk and




(b)  External gill stage of tadpole

  • Just above one day after hatching, the external gill stage
  • Eyes become fully developed and
  • Horny jaws with teeth appear along the rim of
  • Tail elongates and becomes a powerful swimming
  • Pronephric kidneys become fully Frog’s tadpoles are ammonotelic. Nitrogenous waste matter excreted by frog’s tadpole is ammonia.
  • The branchial clefts are perforated and finger-like external gills project from the sides of the head in branchial
  • External gills are three pairs in number; tadpole respires mainly by gills using oxygen dissolved in water.
  • Gut is differentiated into pharynx, oesophagus, lung rudiments, stomach, liver, gall bladder and
  • Tadpole is herbivorous (phytophagus), feeds on aquatic
  • Tadpole has a long coiled intestine because digestion takes place relatively long

(c)  Internal gill stage of tadpole

  • Tadpoles grow older, the hind limb buds and internal gills
  • External gills are replaced by four pairs of internal gills covered with a fold of skin called operculum.
  • Operculum encloses a chamber, opercular chamber, opens to exterior by spiracle.
  • Spiracle is present only on the left side of the
  • During respiration, the water currents enter the mouth, bath the gills of pharynx and exit through the
  • Oral sucker disappear and lateral line receptor This serves to perceive stimuli of movements, currents and vibrations of water.
  • A tadpole of frog resembles a fish in many features except that the tadpole does not possess paired fins and scales on the
  • The tadpole cannot survive when exposed to air because its skin is thin and
  • Exposure of tadpole to land leads to dehydration and
  • First sign of metamorphosis is the appearance of hind
  • End of tadpole in the life history is marked by appearance of

(x) Metamorphosis

  • Metamorphosis is the abrupt transition from larval to adult
  • Metamorphosis includes morphological, anatomical, physiological and behavioural
  • Two or three weeks after breathing with gills, the tadpole larva undergoes drastic changes called
  • Two types of changes during metamorphosis are regressive and progressive.
  • Some of the regressive morphological changes are :

Disappearance of larval sucker, long tail, gill clefts, internal gills and lateral line sense organ.

  • Some of the progressive morphological changes are :
  • Formation of forlimbs breaking through
  • Replacement of pronephros with mesonephric kidney.





  • Enlargement and development of hindlimbs.
  • Development of tongue and
  • Thickening of skin and development of mucous glands.
  • Two chambered heart becomes three
  • Development of
    • During metamorphosis, the disappearance of larval organs is by histolysis and formation of adult organs is by
    • The tail is shortened by reabsorption with the help of lysosomal enzyme cathepsin. This process is also known as autolysis.
    • Nervous system undergoes no special changes (least changes) during
    • Respiratory system undergoes maximum changes during
    • During metamorphosis, skin gets cornified and mucous and poison glands
    • The feeding habit changes from herbivorous habit of tadpole to carnivorous habit of

(xi) Hormonal control of metamorphosis

  • Hormonal control of metamorphosis in amphibian was discovered by Gudernatsch (1912).
  • Metamosphosis occurs only when adequate amount of thyroxine is secreted by thyroid of
  • Since iodine is the main constituent of thyroxine, it is found that deficiency or abundance of iodine in pond water also affect
  • When there is no iodine in pond water or


thyroxine is not secreted in the body, metamorphosis fail to begin. The tadpole continues growing and becomes abnormally large.

  • If thyroxine is given, the tadpoles metamorphose too rapidly giving rise to vary small black frogs which soon
  • If thyroids are removed, giant tadpoles can be reared which are unable to
  • On removing the thyroid from the



If thyroxine is given the tadpoles metamorphose too rapidly giving rise to very small black frogs which soon die

Normal 5-week tadpole







If thyroids are removed giant tadpoles can be reared which are unable to metamorphose


tadpole of frog, it will remain tadpole throughout life.

  • If a small quantity of thyroid extract is added to water in which frog tadpoles are present, it will hasten the
  • The endocrine gland which initiates metamorphosis in frog is
  • Thiourea is antithyroid drug, it inhibits metamorphosis of

Normal-size frog after metamorphosis









Fig : The effect of thyroxine on metamorphosis of tadpoles




  • Neoteny refers to the retention of a larval or embryonic trait in the adult body, g., Cartilaginous skeleton in adult chondrichthyes and larval gills in some adult salamanders.
  • Paedogenesis or paedomorphosis refers to development of gonads and production of young ones by larval or pre adult animal g. liverfluke and ambystoma.
  • Deficiency of iodine in the soil results in the failure of metamorphosis in
  • Axolotl is the larva of ambystoma, it shows

Important Tips

  • Metamorphosis : Transformation of young into a morphologically and physiologically different adult is called metamorphosis. It is of 2 types :
  • Retrogressive metamorphosis : When an advanced larva changes into a degenerate adult g. Herdmania, Sacculina.
  • Progressive metamorphosis : When a simplified larva changes into an advanced adult g. Frog.
  • Embryonic induction : Morphogenetic effect of one embryonic part on another embryonic part. Inducing embryonic part is called inductor or organizer, while responding embryonic part is called responding tissue. This induction is through certain chemicals called evocators.
  • Primary organizers include dorsal lip of blastopore; grey crescent (neural inductor) and chorda-mesoderm (induces forebrain).
  • Secondary organizer g. optic area induces formation of lens.
  • Tertiary organizer g. Lens induces formation of cornea.