REPRODUCTION AND DEVELOPMENT IN ANIMALS PART 3

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).

(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 ooplasm and the plasma membrane (vitelline membrane).

Sperm penetration into ovum also induces following metabolic activities :

(i) The egg surface produces fertilization cone.

(ii) The vitelline membrane is lifted and is converted into fertilization membrane.

(iii) The cytoplasm exhibits movements.

(iv) The permeability of plasma membrane increases.

(v) The rate of protein synthesis increases.

(vi) Mitosis is initiated.

(4) Fusion of gametic nuclei : Entrance of spermatozoan 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. The centriole brought in by the spermatozoan 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.

Chemistry of fertilization

Both egg and sperm contain certain chemicals that are necessary for fertilization these are known as gamones .

The hormones of the sperm are called androgamones and those found in the egg are termed as gynogamones. Two types of Androgamones and Gynogamones are found in gametes.

Table : 8.3-4 Chemicals found in gamete

Gamone	Gamete	Function
Androgamone-I	Sperm	Conserve sperm activity.
Androgamone-II	Sperm	Dissolves vitelline membrane, allowing sperm entrance into the egg.
Gynogamone-I	Ovum	Neutralizes androgamone-I there by increasing sperm activity.
Gynogamone-II	Ovum	Makes sperm head sticky to facilitate attachment of sperm to egg surface.

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.

Significance of fertilization

(i) It provides stimulus for the egg to complete its maturation.

(ii) It activates the ovum to develop into a new individual by repeated mitotic division.

(iii) Fertilization restores the diploid number of chromosomes (46 in man) in the zygote by adding male’s haploid set of chromosomes.

(iv) It makes the egg more active metabolically.

(v) It combines the character of two parents and introduces variations. So help in evolution.

(vi) Sex chromosomes of sperm is either X or Y and helps in sex determination.

(vii) Fertilization membrane formed after sperm entry, checks the entry of additional sperms.

(viii) Copulation path sets the axis of division.

Cleavage

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.

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.

Table : 8.3-5 Different between cleavage and mitosis

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 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, Amia calva and Lepidosteus.

(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 and v^th  cleavage plane of the eggs of Ambystoma maculatum.

(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.

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.

(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.

(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.

(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, tunicates, Amphibia and higher mammals.

Laws of cleavage : Certain fundamental rules or laws of cleavage are following :

(1) Sach’s laws : In 1877, Sach proposed following two laws :

(i) Cells tend to divide into equal daughter cells.

(ii) Each new division plane tends to intersect the preceding plane at right angles. (Acts to maintain the spheroidal shape of blastomeres).

(2) Hertwig’s laws : In 1881, O. Hertwig added following laws of cleavage in Sach’s laws :

(i) The nucleus and achromatic figure (or mitotic spindle) ‘occupy the centre of protoplasmic density’ of the egg or blastomeres in which it lies. Hence, in the microlecithal and isolecithal eggs, the spindle is located centrally; in a telolecithal ovum it is nearer the animal pole.

Corollary : Blastomeres divide into two equal parts unless the yolk is unevenly stored in them.

(ii) The axis of a mitotic spindle occupies the longest axis of the protoplasmic mass in which it lies and division therefore tends to cut this axis transversely (evident in ovoid blastomeres).

Corollary : The ensuing plane of division cuts across the long axis, and the daughter cells revert to a more spheroidal shape.

(3) Balfour’s law : Balfour’s law which was formulated by Balfour in 1885, states that the speed or rate of cleavage in any region of egg is inversely proportional to the amount of yolk or deutoplasm it contains. (In telolecithal eggs, blastomeres at the animal pole divide faster than those nearer the vegetal pole).

(4) Pfluger’s law : The mitotic spindle always elongates in the direction of least resistance.

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

(i) 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

(a) Equal holoblastic cleavage : If the blastomeres are approximately equal, it is called equal holoblastic cleavage.

Examples : Echinoderms, amphioxus and placental mammals.

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

Example : Fish and amphibians.

(ii) 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

(a) 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.

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

Example : Centrolecithal eggs of arthropods.

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. Third cleavage takes place about 72 hours after fertilization.

(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) : 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.

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.

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.

Types of blastula

(i) Coeloblastula : A hollow blastula in which blastocoel is surrounded by either single layered (e.g., echinoderms, amphioxus) or many layered blastoderm (e.g., frog).

(ii) Amphiblastula : It is a type of coeloblastula surrounded by two types of cells. Upper micromeres and lower macromeres. Ex. : frog.

(iii) Stereoblastula : Solid blastula with no blastocoel e.g., in coelentrates annelids and molluscs.

(iv) 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.

(v) 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.

(vi) 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.

Fate mape :

Fate mapping Ist tried by SPECT (1919)

Fate mapping Ist done by W. Vogt. (1929)

Fate mapping is done by vital dyes.

e.g., Neutral Red. Nile blue sulphate, Bismarck brown.

Stain carrier = Agar-Agar and cellophan

Fate mapping can be done in Later blastula or earlygastrula.

Presumptive Areas : Also known as prospective areas.

(1) Animal pole : Animal pole gives rise presumptive ectoderm. It is the region of presumptive epidermis + presumptive neural plate.

(2) Dorsal surface : On dorsally a small area lies in intermediate zone which is the area of Presumptive Notochord.

(3) Laterally : Closed to the notochord, presumptive mesoderm is present.

(4) Vegetal half gives rise presumptive endoderm.

Implantation

Definition : The process of attachment of the blastocyst on the endometrium of the uterus is called implantation.

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.

Gastrulation

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 gastrular movements are found in different animals

(1) 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 frog.

(2) Emboly : It involves inward movement of prospective endodermal and chorda-mesodermal blastomeres from the surface of blastula. Emboly includes following methods :

(i) Invagination : It involves insinking of endodermal cells in the blastocoel to form archenteron. It is found in amphioxus.

(ii) Involution : It involves the rolling in of the chorda-mesodermal blastomeres inside the ectodermal cells over the lips of blastopore. It is also found in the gastrulation of frog.

(iii) Ingression or polyinvagination : In this, individual blastomeres migrate into the blastocoel either from only vegetal pole (called unipolar ingression e.g., Obelia😉 or from all sides (called multipolar ingression e.g., Hydra) to form a solid gastrula called stereogastrula.

(iv) Delamination : It involves splitting off the blastoderm into two layers by the appearance of grooves resulting the formation of hypoblast. It is found in birds.

Formation of layers by gastrulation : Gastrulation includes the formation of following structures

(1) Formation of endoderm : The blastodermic vesicle enlarges and cells present on the lower surface of the embryonal knob detach by delamination from the embryonal knob. The 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.

(b) 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 dispersing. 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 endoderm.

(3) 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 body. Their fate in embryo and adult has been listed below.

Table : 8.3-6 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		Mesentron (Mid gut)
Adrenal medulla		Lining of vagina and urethra
Hypothalamus		Prostate gland

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.

Extra embryonic membrane

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.

(1) 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 layer.

Function : In human beings, it is vestigial. In human embryo it act as the site of blood cell formation until about the 6^th week, when the liver takes over this role.

(2) Amnion : It is formed above the embryo. It consist of outer mesoderm and inner ectoderm. The amnion and the fluid filled amniotic cavity it encloses, enlarge and nearly surround the embryo. Amniotic fluid secreted by both embryo and amnion.

Functions

(i) The amniotic fluid cushions the embryo.

(ii) It protects the embryo from jerk, injury and shocks.

(iii) It prevents desiccation of the embryo.

(3) Allantois : It is a fold of splanchnopleur developed from the hind gut of the embryo. It consist of outer mesoderm and inner endoderm.

Functions

(i) 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.

(ii) The vascular “chorioallantoic membrane” lies in a close proximity to the inner surface of the porous shell. It acts as an extraembryonic lung by supplying the embryo with oxygen.

(4) Chorion : It is outermost fold of somatopleur (outer ectoderm and somatic mesoderm) and surrounds the embryo. 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 mother.

Placenta

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 foetus.

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 wall. 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 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

(1) 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 placenta.

(2) 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 respectively.

(3) It also helps excretion of the embryo as nitrogenous wastes of foetal blood like urea pass into maternal blood through placenta.

(4) 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 defects.

Classification of placenta

(1) According to the foetal membrane involved in the formation of placenta.

(i) Yolk sac placenta : In metatheria or marsupials, such as kangaroo (macropus) and opossum (Didelphys), placenta is derived from yolk sac and chorion.

 

(ii) 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 Eutheria, a primitive allantoic placenta occurs only in perameles (bandicoot) which is a metatherian.

(iii) 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 chorion. However, its mesoderm and blood vessels grow upto chorion whose villi enter the uterine crypts forming chorionic placenta.

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

(i) Epithelio-chorial : Most primitive and simplest type with all six placental barriers.

Examples : Odd hoofed mammals such as horse, ass, pig and lemurs.

(ii) Syndesmo-chorial : Uterine epithelium absent, with five placental barriers.

Examples : Even hoofed mammals such as cow, Buffalow, sheep, goat, camel, Girraffe etc.

(iii) Endothelio-chorial : Uterine epithelium and uterine connective tissues are absent, with four placental barriers.

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

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

Examples : Primates (man, apes and monkey).

(v) Haemo-endothelial : Foetal capillaries indirect contact with maternal blood, only one placental barrier.

Examples : Rat, guinea pig and rabbit.

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 recognized.

(i) Non deciduous placenta : In most mammals villi are simple, unbranched and merely opposed without intimate contact between foetus and uterine wall. At the time of birth or parturition, villi are easily withdrawn from maternal crypts without 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.

(a) Diffuse : Villi remain scattered all over the surface of allantochorion. e.g., pig, horse, lemur.

(b) Cotyledonary : Villi are arranged in separate tufts or patches called cotyledons. e.g., goat, sheep, cow, deer.

(iii) Intermediate : Villi are arranged in cotyledons as well as scattered. e.g., camel, giraffe.

(ii) 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

(a) Zonary : Villi form an incomplete (e.g., racoon) or complete girdle encircling the blastocyst. e.g., cat, dog, seal, Lion, Tiger, Elephant etc.

(b) Discoidal : Villi are restricted to a circular disc or plate on the dorsal surface of blastocyst. e.g., insectivores, bats, rodents (rat, mouse), rabbit, bear.

(c) 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 apes.

(iii) 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 e.g., bandicoot (perameles), mole (Talpa).

Gestation period and parturition

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

Table : 8.3-7 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 (36 weeks)
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).

Development in frog

(1) Breeding : Frog breeds in the rainy season, June to September. Male frogs produce crocking sound (mating call) by their vocal sacs.

(2) Ovulation : The eggs in the stage of secondary oocytes are released into the body cavity by rupture of ovary during ovulation.

(3) Spawning : Spawning is the act of laying of eggs by the female frog stimulated by the male during amplexus.

Spawn is a cluster or mass of eggs laid by a female. a spawn of Rana tigrina contains about 3000-4000 eggs.  Gelatin protects the egg from predators and also acts as an insulator keeping the egg warm.

(4) Fertilization : Fertilization in frog is external taking place in water. When a sperm enters into the egg of frog, second meiotic division occurs. Sperm entrance point marks the anterior side of the future embryo. Gray crescent marks the dorsal side of the future embryo.A gray crescent appears in the equitorial zone geometrically opposite to the sperm entrance. The bilateral organization is established at the time of sperm penetration. The region where sperm enter the egg cell is called ‘reception cone’.

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 predators.

(5) Cleavage : First cleavage of frog is meridional passing through median longitudinal axis, holoblastic and equal. Gray crescent is present only in two blastomeres of future dorsal side.

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).

Smaller cells are micromeres and larger ones are called macromeres. The micromeres contain no yolk and macromeres contain large amount of yolk.

Fourth cleavage involves two synchronous meridional divisions resulting in the formation of 16 blastomeres.

Fifth cleavage involves two simultaneous latitudinal divisions resulting in the formation of 32 blastomeres.

After sixth or seventh cleavage division, the embryo looks like a mulberry-shaped ball of cells. This called morula stage.

(6) Morula : It  is a solid ball of cells. A cavity called blastocoel appears towards animal hemisphere. The blastula of frog is called coeloblastula.

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 normal.

(7) Gastrulation : 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.

Invagination of prospective endoderm cells occurs below equator, exactly below the midpoint of gray crescent of blastula. It results in the formation of a slit later giving rise to blastopore.

Posterior side of future tadpole is represented by the side of frog’s embryo bearing the yolk plug.

(8) 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 work.

(9) Neurulation : Neurulation takes place after gastrulation. 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 folds. Neural folds meet and fuse at the mid dorsal line. Neurulation includes the formation of neural tube, notochord and gut.

 Formation of notochord is known as notogenesis.

(i) Post neurular development :The development takes place inside egg membrane upto tail bud larval stage.

Hatching occurs in 6^th day of embryonic life. During hatching, the young frog is called tadpole larva. Newly hatched tadpole larva remain attached to aquatic plants by its oral sucker.

After 24 hours of hatching, mouth and anus are perforated. The larval body is elongated forming head, trunk and tail.

(ii) External gill stage of tadpole

(a) Just above one day after hatching, the external gill stage starts.

(b) Eyes become fully developed and functional.

(c) Horny jaws with teeth appear along the rim of mouth.

(d) Tail elongates and becomes a powerful swimming organ.

(e) Pronephric kidneys become fully developed. Frog’s tadpoles are ammonotelic. Nitrogenous waste matter excreted by frog’s tadpole is ammonia.

(f) Tadpole is herbivorous (phytophagus), feeds on aquatic plants.

(g) Tadpole has a long coiled intestine because digestion takes place relatively long time.

(iii) Internal gill stage of tadpole

(a) Tadpoles grow older, the hind limb buds and internal gills develop.

(b) External gills are replaced by four pairs of internal gills covered with a fold of skin called operculum.

(c) Operculum encloses a chamber, opercular chamber, opens to exterior by spiracle.

(d) Spiracle is present only on the left side of the tadpole.

(e) During respiration, the water currents enter the mouth, bath the gills of pharynx and exit through the spiracle.

(f) Oral sucker disappear and lateral line receptor develop. This serves to perceive stimuli of movements, currents and vibrations of water.

(g) A tadpole of frog resembles a fish in many features except that the tadpole does not possess paired fins and scales on the body.

(10) Metamorphosis

(i) Metamorphosis includes morphological, anatomical, physiological and behavioural changes.

(ii) Two or three weeks after breathing with gills, the tadpole larva undergoes drastic changes called metamorphosis.

(iii) Two types of changes during metamorphosis are regressive and progressive.

(iv) Some of the regressive morphological changes are :

(v) Some of the progressive morphological changes are :

(vi) Formation of fore limbs breaking through operculum.

(vii) Replacement of pronephros with mesonephric kidney.

(viii) The tail is shortened by reabsorption with the help of lysosomal enzyme cathepsin. This process is also known as autolysis.

Hormonal control of metamorphosis

(1) Hormonal control of metamorphosis in amphibian was discovered by Gudernatsch (1912).

(2) Metamosphosis occurs only when adequate amount of thyroxine is secreted by thyroid of tadpole.

(3) Thiourea is antithyroid drug, it inhibits metamorphosis of frog.

(4) Neoteny refers to the retention of a larval or embryonic trait in the adult body, e.g., Cartilaginous skeleton in adult chondrichthyes and larval gills in some adult salamanders.

(5) Paedogenesis or paedomorphosis refers to development of gonads and production of young ones by larval or pre adult animal e.g., liverfluke and ambystoma.

(6) Deficiency of iodine in the soil results in the failure of metamorphosis in ambystoma.

(7) Axolotal is the larva of ambystoma, it shows paedogenesis.

?  Smallest sperm is of Crocodile (.02mm) and largest sperm is of discoglossus (2mm)

?  Complete spermatogenesis in man takes place in 74 days.

?  In 1ml of human semen 100 million sperms are present.

?  Infertility which arises due to less number of sperms is called Oligospermia.

?  The condition of presence of normal number of motile sperms in human semen is termed as isozospermia.

?  The condition of presence of completely non-motile sperms in human semen is termed as Necrospermia.

?  Smallest eggs are of humming bird.

?  In lower animals large amount of eggs are poduced because their chances of survival are very less.

?  The asexual process replaced by the sexual method is known as apomixis.

?  No natural death in organisms showing binary fission e.g., Amoeba, so are called immortal.

?  The croaking sound made by frog is sex call for female partner.

?  Leuvenhock (1677) saw human sperm.

?  In frog bidder canal help in sperm passout.

?  Androgenesis : Development in which embryo has only paternal chromosomes, male parthenogenesis.

?  Gynogenesis : Development in which embryo has only maternal chromosomes, female parthenogenesis.

?  Castration / Orchidectomy : Removal of testes. It produce eunuchs. Castration changes aggressiveness of male into docile nature.

?  Prostatitis : Inflammation of prostate gland. Prostate cancer is common in ageing males.

?  Human egg : 0.1 mm in diameter.

?  Peculiar spermatozoa : Ascaris has amoeboid spermatozoa devoid of flagellum. Some crustaceans also have atypical sperms.

?  Sperms form about 10% of the ejaculated semen.

?  Protandry : Spermatozoa mature earlier than ova in bisexual animals e.g., – Hydra, Earthworm.

?  Andrology : Branch of medicine concerned with diseases peculiar to male sex.

?  Spermatophore : A capsule containing spermatozoa, as in cuttle fish and salamander.

?  Oophoritis : Inflammation of an ovary.

?  Vitellogenesis : Process of laying down of yolk in the primary oocyte. It occurs in the prophase of meiosis-I.

?  Metagenesis : Alternation of sexually and asexually reproducing forms in the life cycle of an animal e.g., Obelia.

?  Protogyny : Ova mature earlier than sperm in a bisexual animal e.g., Herdmania.

?  Spermathecae : Small sacs that form a part of female reproductive system of earthworm and store spermatozoa received from the male for use in future.

?  Ovipositor : A specialised female organ for laying eggs, specially in insects.

?  Rutting season : It is a brief period of pronounced sexual activity in males.

?  Tubectomy (Salpingectomy) : Surgical removal of oviducts.

?  Von bear : Discovered ovum.

?  Strobilation : Asexual multiplication by transverse fusion and is found in Scyphistoma of Aurelia and also found in Taenia.

?  Richard owen gave term parthenogenesis.

?  Testes are also called spermaries.

?  Vaginal coelom : Cavity of scrotal sac.

?  To-gene : Testicular organisation gene located on Y-chromosome and is a male determining factor.

?  Uterus : It is also called womb.

?  Vestibule : Acts as a urinogenital sinus.

?  Perineum : Area between the fourchette and anus.

?  Bartholin’s or Bulvo vestibular glands of female homologous to Cowper’s glands of male.

?  Precocious puberty : Puberty attained before the normal age.

?  Hypermastia : More than normal number of breasts.

?  In seasonally breedings animals, testes show testicular cycle.

?  Spermatogenesis is continous process, while oogenesis is a discontinuous process.

?  In spermatogenesis, spermatogonium produces four sperms while in oogenesis, one oogonium produces one ovum and 2 or 3 polar bodies.

?  Golgi rest : Part of golgi body which is lost during spermiogenesis.

?  Yolk nucleus : Also called Balbiani body. A mass of mitochondria and golgibody near the nucleus which controls vitellogenesis.

?  Redundancy : Gene amplification of r-RNA genes for rapid RNA and protein synthesis.

?  Ring centriole : Also called annulus or Jensen’s ring.

?  Menstruation is also called “Weeping of uterus for the lost ovum or funeral of unfertilized egg”.

?  Menstrual cycle is associated with withdrawal or progesterone.

?  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.

?  H. Spemann and Mangold : Reported embryonic induction on newt and gave concept of primary organizers.

?  Pander : Formation of three germinal layers in chick embryo.

?  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.

?  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.

?  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.

?  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-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.

?  First embryonic membrane to be formed is endoderm.

?  Cells of Rauber : Those cells of trophoblast which are in contact with embryonal knob.

?  Protostomous : When blastopore forms the mouth in development and is found in non-chordates except echinoderms, hemichordates and chordates.

?  Deuterostomous : When blastopore forms the anus in development. Mouth is developed later than anus e.g., echinoderms hemichordates and chordates.

?  Zona pellucida disintegrates just after completion of cleavage.

?  Cells of corona radiata disperse just before implantation.

?  Teratogens are those physical, chemical and biological agents, which may cause malformations in the developing embryo.

?  Postpartum care : Care after childbirth.

?  Lanugo : Most of the body of foetus is covered with downy hairs called lanugo which are generally shed before birth

?  Uterine milk : Nutritive endometrial secretion.

?  Nidiculous or Altricial young : Underdeveloped and helpless young born e.g., cats, dogs, rats, etc.

?  Retrogressive metamorphosis : When an advanced larva changes into a degenerate adult e.g., Herdmania, Sacculina.

?  Progressive metamorphosis : When a simplified larva changes into an advanced adult e.g., Frog.

?  Primary organizers include dorsal lip of blastopore; grey crescent (neural inductor) and chorda-mesoderm (induces forebrain).

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