REPRODUCTION AND DEVELOPMENT IN ANIALS 2

(iii) Mucous membrane : It is the innermost layer. This layer is made up of ciliated columnar epithelium and the connective tissue.

The oviduct shows 4 regions :

(a) Infundibulum : It is the broad, funnel-shaped proximal part. Its margin bears motile, finger-like processes called fimbriae. It opens into the body cavity by an aperture called ostium. The latter lies close to the ovary to receive the egg released from the ovary. The fimbriae bear cilia which beat toward the ostium to direct the egg into the infundibulum.

(b) Ampulla : It is the long, wide, thin-walled, tortuous major part of the fallopian tube next to the infundibulum. Ampulla is site for fertilization.

(c) Isthmus : It is the very short, narrow, thick-walled, straight part that follows the ampulla.

(d) Uterine part : It is also narrow and passes through the uterine wall, and communicates with the uterine cavity.

(3) Uterus  : It is pyriform, hollow muscular thick-walled but distensible median structure located above and behind urinary bladder that is meant for nourishing and development of foetus. For this uterus is capable of tremendous enlargement. The empty uterus is 7.5 cm long and 5 cm broad and 2.5 cm thick. Lining layer of uterus, called endometrium (mucous membrane), consists of an epithelium and lamina propria of connective tissue. Epithelium is a mixture ciliated and secretory columnar cells. Lamina propria contains tubular glands, fibroblasts and blood vessels.

Histologically, uterus consist of 3 layers of tissues perimetrium, myometrium, and endometrium. Perimetrium composed of simple squamous epithelia. Endometrium, is highly vascular composed of ciliated columnar epithelia.

The normal position of the uterus is anteflexed, that is, it is bent forward on itself at the level of the internal os so as to lie almost horizontally over the bladder. The cervix is composed largely of the biggest and the most powerful sphincter muscle in the body. It is strong enough to hold about 7 kg. of foetus and fluid in the uterus against the pull of gravity during pregnancy.

The cavity of the uterus can expand 500 times during pregnancy, from 10 cm³ to 5,000 cm³.

Types of uterus

(i) Duplex : These are the simplest type of uteri. In it both the uteri are completely separated and open independently into the vagina through two separate openings e.g., Rat.

(ii) Biparite : In these uteri, the lower part of the two uteri are fused and there is a septa in between the two, e.g., Carnivore mammals.

(iii) Bicornuate : The lower parts of the 2 uteri are fused with each other but the partition wall is absent, e.g., Rabbit.

(iv) Simplex : When both the uteri are completely fused with each-other to form only one structure, these are the most developed uteri e.g., Man.

Functions of uterus : The uterus plays multiple role. The uterus is site of menstruations, implantation of a fertilized ovum, development of foetus and labour.

(4) Vagina : It is tubular female copulatory organ, passageway for menstrual flow as well as birth canal of about 10 cm length between external opening (vaginal orifice) in vestibule and cervix with depression or fornix around cervix, two longitudinal ridges and numerous transverse folds or vaginal rugae. Vaginal wall is made of an internal mucosa, muscular layer and an outer adventitia. Its mucous membrane is nonkeratinised stratified squamous epithelium. Glands are absent. However, cervical glands do pass on some mucus into it during ovulation. The epithelial cells contain glycogen (from puberty to menopause) which shows cyclic changes which due to decomposition produce organic acid. Certain bacteria (species of Lactobacillus and Lactoneustroc, also called Doderlein’s Bacillus) bring about fermentation and produce acid which inhibits growth of other microorganisms. In virgins the vaginal orifice is partially covered by an annular centrally perforate membrane called hymen.

(5) External genitalia / vulva : There is a depression, the vestibule, in front of the anus. It is flanked by two pairs of fleshy folds of skin : the inner small, thin, moist, labia minora and outer larger, hair-covered labia majora. All the labial folds have numerous sebaceous and sweat glands on both sides. A small erectile organ, the clitoris, lies at the anterior junction of the labia minora. It is homologous to the penis in the male but is very small and solid, having no passage through it. It consists of a short shaft with erectile tissue. It ends in a rounded glans clitoridis. The latter is covered by a small hook of skin, the prepuce. Rubbing of clitoris during intercourse produces a pleasurable sensation. This seems to be its only function. A membranous fold, called fourchette, connects the posterior ends of the labia minora. The area between the fourchette and the anus is termed perineum. Urethra and vagina open by separate apertures, the upper urethral and lower vaginal orifices, into the vestibule. The vaginal orifice is normally covered in the virgin by a membrane, the hymen. A slit in the hymen allows menstrual flow to pass out. The hymen is ruptured during first copulation, or due to some other reason. The hymen presumably has a protective role, but is absent in many women. A fleshy elevation above the labia majora is known as mons veneris or mons pubis. It bears pubic hair, made up of adipose tissue.

(6) Bartholins or vestibular glands : They are a pair of small glands which open in the vestibule lateral to vaginal orifice. The secretion is thick, viscid and alkaline for lubrication and counteracting urinary acidity (similar to Cowper’s glands in males).

(7) Breasts / Mammary glands : The breasts are rounded eminences located over the pectoral muscles on the front wall of the thorax. These enlarge considerably in the adult female. Each breast has near its middle a nipple surrounded by a circular, pigmented (deep pink to brown) area called areola. The breasts contain fatty and connective tissues and mammary (milk) glands. The latter are compound saccular glands and are modified sweat glands. The connective tissue supports the mammary glands and the adipose tissue covers them. The glands open on the nipples, the lactiferous ducts. Just under the nipples, the lactiferous ducts widen to form lactiferous sinuses, to store milk during lactation. A nursing mother produces 1 to 2 litres of milk per day.

Milk secretion is under the control of prolactin (of anterior pituitary) while milk ejection is under control of oxytocin (of posterior pituitary).

First or premilk after parturition is called colustrum.

Onset of puberty in the females : Woman attains puberty about the age of 13. Its onset it triggered by the production of the anterior pituitary’s follicle-stimulating hormone (FSH) which promotes growth of the ovarian follicles. The follicles then secrete the hormone estrogen from the follicle cells in the ovaries. This hormone brings about the growth and maturation of the reproductive tract and the development of accessory sex character. The latter include growth of breasts and external genitalia, broadening of pelvis, growth of pubic and axillary hair, increase in the subcutaneous fat, particularly in thighs, shoulders, buttocks and face to give a typical feminine contour and initiation of menstruation and ovulation.

Hormonal control of female reproductive system

         Ovary is regulated by pituitary gonadotropins or G_nRH. Anterior pituitary secretes follicle stimulating hormone (FSH) which controls of the transformation of young primary follicle into Graffian follicle, maturation of ovum and secretion of estrogens by its follicular cells. The Luteinizing Hormone (LH) of anterior pituitary regulates the ovulation from the Graffian follicle, transformation of empty Graffian follicle into yellowish, conical corpus luteum and secretion of progesterone hormone from the corpus luteum.

Growth and function of secondary sex organs are regulated by estrogens and progesterone. Estrogens control the growth, maintenance and functioning of secondary sex organs of female. Progesterone suspends ovulation during pregnancy, promotes implantation of foetus on the endometrium and development of foetus in the uterus.

At the end of pregnancy, the corpus luteum secretes relaxin which broadens the pelvis for easy parturition.

Reproductive cycle in female : The events involved in reproduction in female mammals occur in a cyclic manner. Constituting the reproductive cycle or ovarian cycle. The reproductive cycle is of two types :

(1) Oestrous cycle                            (2) Menstrual cycle

(1) Oestrous cycle : The oestrous cycle consists of a few days of oestrus or “heat” followed by a few days of anoestrus of “quiescence”.

During oestrus, the female is sexually responsive, allows a male to copulate, eggs are released and pregnancy is possible. During anoestrus, the female become passive and does not accept a male. The oestrus occurs in most species of mammals. Many mammals reproduce in the breeding season only. The oestrus cycles run only during the breeding season in these mammals and anoestrus spreads over the entire non breeding season.

Except primates, oestrous cycle is found in all mammals.

Some mammals, such a cow and buffalo experience oestrous cycles throughout the year. They have no specific breeding season.

In rabbit the oestrous cycle is of 7 days.

Oestrous cycle of rat is of 5 days only.

Dog has one cycle per year.

Oestrous is also found in the New world monkeys.

During the oestrous-cycle, the wall of uterus does not dissolve i.e., no bleeding takes place.

(i) Mono – oestrous animals : In the breeding period of some animals only one oestrous cycle is present. e.g., Rabbit, Hare, Dog, Fox, Bat, Deer etc.

(ii) Poly – Oestrous animals : In many animals many oestrous cycles are found in the breeding period. e.g., – Rats squirrels, Cow, Sheep, Pig, Horse etc.

(2) Menstrual cycle : The gamete formation in females is a cyclic activity that takes about 28 days and involves changes in the structure and function of the entire reproductive system. It is called menstrual cycle.

The menstrual cycle occur only in primates, except new world monkey.

The length of menstrual cycle varies widely in women, but on average it is completed in 28 days (mensem means a month). In a female, successive cycles may vary in length by 1 to 2 days. It is absent during pregnancy, may be suppressed during lactation and permanently stops at menopause.

Menstrual cycle is divided into four phases –

(i) Follicular (Proliferative) phase or Post-menstrual or Pre-ovulatory phase : It follows the menstrual phase and lasts for about 9-10 days (from 6 to 13^th day of menstrual cycle). It involves following changes :

(a) Under the stimulation of FSH-RF of hypothalamus, there is increased secretion of FSH from anterior pituitary.

(b) FSH stimulates the change of a primary follicle of the ovary into a Graffian follicle.

(c) Follicular cells of Graffian follicle secrete estrogens.

(ii) Ovulatory phase or fertility phase : It involves the ovulation from the Graffian follicle of ovary. The mature graffian follicle rises to the surface of the ovary and ruptures to release ovum. The phenomina is called ovulation. It occurs midway between two menstrual cycles on 14^th day of the onset of the menstrual cycle. it is caused by increasing turgidity and contraction of smooth muscles fibres around the Graffian follicle. Ovum is received by the fimbriad of the fallopian tube. Ovum is viable for two days. Ovulation is controlled by the increased level of LH in the blood. Egg at that time is in the secondary oocyte state. LH also starts the change of empty Graffian follicle into corpus luteum and secretion of progesterone from corpus. During ovulation, the secondary oocytes remains surrounded by its zona pellucida and corona radiata.  There is no much change in uterine endometrium during ovulatory phase. In animals the ovulation follow three patterns :

(a) Fix or spontaneous ovulators : In these animals ovulation takes place a fix time in the midway of cycle. There is no need of coitus for ovulation. e.g., Primates (Human, Ape and Monkey)

(b) Induced or reflex ovulators : In these animals copulation or ciotus is necessary for ovulation. e.g. – Rabbit.

(c) Seasonal ovulators : Ovulation occur in breeding season e.g., – Frog.

(iii) Luteal or progestational or Pre-menstrual or Secretory or Post-ovulatory phase : It lasts for about 12 – 14 days and extends from 16^th to 28^th day of menstrual cycle. It is characterised by following changes –

(a) Corpus luteum (Yellow body) : Formed from empty Graffian follicle, increase in size, so is called luteal phase.

(b) Corpus luteum begins to secrete hormone called progesterone. The latter reaches its peak about 22^nd day after the beginning of cycle.

(iv) Menstrual phase or bleeding phase : It lasts for about 3 – 5 days and extends from 1^st to 4^th day of the menstrual cycle. When the ovum remains unfertilized, then the corpus luteum starts degenerating. The level of progesterone in the blood declines. The uterine tissues fail to be maintained. Then the unfertilized ovum along with ruptured uterine epithelium, about 50 – 100 ml of blood and some mucus is discharged out through the vaginal orifice and is called menstrual flow or menstruation.

Decrease in the level of progesterone and estrogens in the blood stimulates the hypothalamus and anterior pituitary to release FSH-RF and FSH respectively (Positive feedback). FSH starts the follicular phase of next menstrual cycle.

Effect of fertilization : If fertilization occurs and foetus is implanted in the endometrium, the trophoblast cells of the developing placenta secrete a hormone human chorionic gonadotrophin (hCG). This hormone, like LH, maintains the corpus luteum and the secretion of progesterone and estradiol by it. These two hormones check the breakdown of the endometrium of the uterus. the absence of menstrual bleeding (the ‘period’) is the earliest sign of pregnancy. By the 16^th week of pregnancy, the placenta produces enough progesterone and estradiol for a normal pregnancy, and the now unnecessary corpus luteum undergoes shrinkage. Fertilization restore diploid condition and equatorial division is completed only if fertilization occurs.

Miscarriage : Prematuredegeneration of corpus luteum is the common cause of miscarriage at about 10 – 12 week of pregnancy (miscarriage means abortion)

Menopause (Climacteric period)

Definition : It is the period when ovulation and menstrual cycle stop in human female.

Period : Between 45 to 55 years. The average period of menopause is currently 52 years. In some, this occurs gradually (in between a period of 1-5 years) while in some this occur abruptly.

Characters : Menopause is characterized by hot flushes (sensation of warmth spreading from the trunk to the face) and a number of psychic symptoms. FSH is secreted in the urine.

Cause : Decline in the estrogen level.

• The function of the testes declines slowly with advancing age, especially in their late 40 yrs or 50 yrs due to decrease in testosterone secretion and is called male climacteric.

Disorders of female reproductive system

(1) Sterility : Inability of the female to conceive, due to inadequacy in structure or function of the genital organs.

(2) Menstrual irregularity

(i) Amenorrhoea : Absence of menstruation.

(ii) Hypermenorrhoea : Excessive or prolonged bleeding of uterus.

(iii) Dysmenorrhoea : Painful menstruation.

Pregnancy test : During pregnancy, hCG may be detected in the urine, and this forms the basis of pregnancy test.

• Oral contraceptive checks ovulation and implantation (for more information see endocrine system). Oral contraceptives with high concentration of progesteron and less concentration of estrogen.

Gametogenesis

The process of the formation of haploid gametes from the undifferentiated, diploid germ cells in the gonads for sexual reproduction is called gametogenesis.

The process of Gametogenesis is stimulated by the FSH or Follicle Stimulating Hormone and for this process Vitamin “A” and “E” are also necessary.

As a result of this process, male gamete sperm and female gamete egg is formed.

Types of gametogenesis

(1) Spermatogenesis

The process of formation of sperms in seminiferous tubules of the testis of the male animal is termed as spermatogenesis.

In mammals, testis have several coiled tubules in it called the seminiferous tubules. Sperms are formed in these tubules. The inner wall of seminiferous tubules is made up of germinal epithelium whose cells are cuboidal.

The endodermal cells of yolk sac migrate in testes and become primordial germ cells. Due to the division of these cells sperms are formed.

Some large cells are also found in this germinal epithelium. These are called the “Sertoli cells or Sustentacular cells”. These cells provide nutrition to the maturing sperms in the form of Glycogen. For getting nutrition, the head of the sperms are submerged in the cytoplasm of sertoli cells.

Sertoli cells mainly provide nutrition and conserve the various stages of spermatogenesis. Spermatogenesis is a continuous process. To make it easier for study, it has been divided into the following steps –

(i) Formation of spermatid.

(ii) Spermiogenesis or Spermateleosis.

(i) Formation of spermatids : This process begins as the animal attains sexual maturity. The endodermal cells of the yolk sac which participate in this process are termed as the primordial germ cells. The process of formation of spermatids from primordial germ cells are termed as spermatocytosis. It has 3 sub-stages –

(a) Multiplication phase : During this process the primordial germ cells repeatedly undergo mitosis division, and as a result of these divisions spermatogonia are formed. Spermatogonia are diploid.

(b) Growth phase : Some spermatogonia either due to growth or due to food storage become 2 or 3 times of their original size, and are now known as primary spermatocytes. The remaining spermatogonia remain in the seminiferous tubules in the form of reserved stock. The primary – spermatocytes formed during the growth phase are diploid. Growth phase is the longest.

(c) Maturation phase : Primary – spermatocytes undergo Meiosis-I and as a result 2 haploid secondary spermatocytes are formed. This division is termed as First Maturation Division or Reductional division. Secondary spermatocytes undergo Meiosis II or equational division, and as result, 2 spermatids are formed from each secondary spermatocyte. Thus, from 1 diploid primary spermatocytes 2 secondary spermatocytes are formed on meiosis I and from 2 haploid secondary spermatocytes 4 spermatids are formed on meiosis-II. Metamorphosis of spermatids into sperms in known as Spermiogenesis or Spermatoliosis.

(ii) Spermatoliosis : The process of transformation of a round non-motile and haploid spermatid obtained from spermatocytosis into thread-like, motile and haploid sperm is termed as spermatoliosis. From different parts of the spermatid different parts of the sperm are formed. These are as follows –

(a) From nucleus and golgibody ® Head part

(b) From mitochondria ® Middle part

(c) The structure of the head of the sperm mainly depends on the structure of the nucleus. During spermatoliosis, nucleus contracts and acquires different shapes.

Structure of sperm

Structure of sperm has three parts

(1) Head          (2) Middle piece     (3) Tail

(1) Head : It is flat and oval in human sperm. It is composed of a large posterior nucleus and a small anterior acrosome.

Acrosome is formed from the golgi complex. It contains digestive enzyme hyaluronidase and proteinase. It is the capitis covering above the nucleus. It is surrounded by double membrane. Acrosome + its membrane are together called Galea-capatis. Acrosome plays important role in penetration of ovum by sperm.

Remaining part of the head is nucleus. Narrow space between the nucleus and the acrosome is termed as “perforatorium”. Nucleus of the sperm is very small. In it nucleoplasm and nucleolus are absent. It contains only chromatin. At the base of the nucleus in a pit like depression proximal centriole is present. In between the head and the middle piece a small neck is present. In this neck part a distal centriole is located. Both the centrioles are at right angles to each other. Proximal centriole first induce cleavage in a fertilized egg. First spindle fibre forms from it.  Distal centriole gives rise to the axial filament of the sperm. It has (9+2) microtubular arrangement.

(2) Middle piece : This is known as the energy-chamber of the sperm. Many mitochondria spirally surround the axonema, this is called “Nabenkern sheath”. This part provides energy to the sperm for locomotion. In middle-piece, cytoplasm is found in the form of a thin-sheet called Manchett. In middle-part, axonema is surrounded by 9 solid fibres made up of proteins. At the posterior end of the middle-piece a Ring centriole is found. Its function is not known.

(3) Tail : The longest and the fibrous part of the sperm is termed its tail.

Sperm moves with the help of its tail. Basal granule of the tail is Distal centriole. Tail has 2 parts

(i) Main part : This part is broad. It contains cytoplasm and is surrounded by 2 solid fibres.

(ii) End piece : This part is narrow in it cytoplasm is absent only axonema is present. In it solid fibres are also absent. In the sperm of certain animals, tail is absent. e.g.,

(a) Ascaris : Tailless, ameboid sperms

(b) Cray fish : Tailless, stellate (star shape) sperms.

(c) Crab and lobser : Tailless sperms with 3 spines at apex.

(d) Biflagellage sperms : In Toad fish (Opsanus)

(e) In Opposum : Many sperms fuse together by their heads to form a “sperm-boat”.

(f) Gastrapods have hexaflagellated sperms.

Oogenesis

Oogenesis takes place in the ovaries. Unlike sperm formation that starts at puberty, egg formation begins before birth but is completed only after fertilization. Oogenesis consists of three phases –

(a) Multiplication phase

During foetal development, endodermal cell of yolk sac enter into ovary and begins oogenesis.

These cells undergo mitotic divisions, producing undifferentiated germ cells called oogonia or egg mother cells in the ovary. The oogonia have diploid, number of chromosome, 46 in humans. The oogonia multiply by mitotic divisions and produce ovigerous cords or egg tubes of pfluger in mammals.

(b) Growth phase : It is prolonged and slow. Oogonia form rounded masses or egg nests at the tips of egg tubes of pfluger.

An egg nest forms ovarian follicle (Graffian follicle) one central oogonium grows and functions as primary oocyte. The others form the covering follicular cells. the latter provide nourishment to primary oocyte. Some nourishment also comes from outside. Yolk is deposited in this state. This phenomenon is called vetellogenesis.

In cooperation with follicular cells, the enlarged primary oocyte secrete mucoprotein membrane or zona pellucida outside its own plasma membrane or vitelline membrane. There is increase in reserve food, size of nucleus, number of mitochondria; functioning of golgi apparatus and complexing of endoplasmic reticulam.

(c) Maturation phase : Meiosis occurs. Nucleus shifts towards animal pole and undergoes meiosis – I. A daughter nucleus alongwith small quantity of cytoplasm is extruded as primary polar body or polocyte below zona pellucida. Simultaneously primary oocyte is changed into haploid secondary oocyte. It proceeds with meiosis – II but stops at metaphase-II. Ovum is generally shed in secondary oocyte stage.

After fertilization, the second meiotic division is completed with unequal cytoplasmic cleavage. This forms a large cell the ootid with essentially whole of the cytoplasm, and a very small cell, the second polar body. The ootid and the second polar body are haploid as the second meiotic division is equational. The first polar body may divide at about the same time into two polar bodies. One primary oocyte forms, after two meiotic division, one haploid ootid and two or three haploid polar bodies. The ootid grows into a functional haploid ovum.

Structure of ovum

An ovum is generally spherical, nonmotile gamete with yolky cytoplasm and enclosed in one or more egg envelops. Size of ovum varies in different animals and depends upon the amount of yolk. Size of ovum varies from 10m to a few cm. Largest sized egg is of ostrich and is about 170 ´135 mm. Egg size and yolk amount are interdependent. It is about 50m in many polychaete worms, 150m in tunicates but very large sized in birds and reptiles. In mammals, it is generally microlecithal and about 100m.

Human ovum is microlecithal with large amount of cytoplasm. Cytoplasm is differentiated into outer, smaller and transparent exoplasm or egg cortex and inner, larger and opaque endoplasm or ooplasm.

Egg envelopes. Human ovum is surrounded by a number of egg envelopes :

(a) Vitelline membrane : It is inner, thin, transparent and is secreted by ovum itself.

(b) Zona pellucida :  It is middle, thick, transparent and non-cellular.

(c) Corona radiata : It is outer, thicker coat formed of radially elongated follicular cells. Between the vitelline membrane and zona pellucida, there is a narrow perivitelline space.

Table : 8.3-3 Differences between Spermatogenesis and Oogenesis

S.No.	Characters	Spermatogenesis	Oogenesis
1.	Site of occurrence	In the seminiferous tubules of testes.	In the ovaries.
2.	Total period	It is a continuous process and completed in 74 days in humans	It is a discontinuous process and completed in a minimum 12-15 yrs.
3.	Growth phase	Of shorter duration	Of longer duration
4.	Yolk synthesis	No yolk is synthesized in growth phase	Vitellogenesis occurs in growth phase.
5.	Nuclear changes	Nucleus becomes condensed by the loss of superfluous materials.	Nucleus is bloated due to increase in necleoplasm.
6.	Number of gametes	One spermatogonium forms 4 haploid sperms.	One oogonium forms only one haploid ovum.
7.	Polar bodies	Not formed.	Two or three polar bodies are formed.
8.	Site of completion	It is started and completed within the testes.	It is started inside the ovary but is generally completed outside the ovary, into oviduct.
9.	Size of gametes formed	Sperm is much smaller than spermatogonium.	Ovum is much larger than oogonium.

Types of eggs

(1) On the basis of amount and distribution of yolk

(i) Alecithal or Microlecithal or Oligolecithal or Meolecithal and Isolecithal or Homolecithal : The amount of yolk is very small in these types of eggs. (Oligolecithal or Microlecithal or Alecithal) and yolk is evenly distributed in these eggs (Isolecithal or Homolecithal). Examples – Egg of Amphioxus, Eutheria (Human egg), Metatheria and Sea-urchin.

(ii) Mesolecithal or Telolecithal eggs : In this type of egg the amount of yolk is moderate and yolk is concentrated in the basal part of egg (telolecithal egg). Examples – Egg of Amphibia, Petromyzon and Lung fishes.

(iii) Polylecithal or Macrolecithal or Megalecithal eggs : Eggs are with large amount of yolk e.g., eggs of shark, bony fish, Reptiles, birds, prototherian, concentrated mainly in vegetal pole.

In discoidal or highly telolecithal eggs, the yolk is enormous in amount and cytoplasm is confined to a disc like area on yolk. This disc of cytoplasm is called germinal disc. Example – Eggs of reptiles, birds, protherian mammals.

(iv) Centrolecithal : Yolk concentrate in centre e.g., Insects egg.

Smallest eggs are of 50m in the polychaeta and the largest eggs are of an ostrich.

(2) On the basis of fate

(i) Determinate / Mosaic eggs : Every part of fertilize egg has a definite fate, so that fate of every blastomere is determined from the beginning. It is found in invertebrates except echinoderms.

(ii) Indeterminate / Regulative eggs : The fate of different parts of egg or its blastomeres is not predetermined. Example – Echinoderms, Vertebrates.

(3) On the basis of shell

(i) Cleidoic eggs : Eggs surrounded by a hard shell are known as cleidoic eggs. These eggs are found in those animals which have a terrestrial mode of life of which lay eggs on land. These eggs have more amount of yolk. These are adaptations to terrestrial mode of life. Shell prevents the egg from dessication. e.g., – Eggs of “Reptiles”. “Birds”. “Insects” and “Prototherians”.

(ii) Non – Cleidoic eggs : Eggs which are not surrounded by a hard shell are called Non-cleidoic eggs. These eggs are found in all oviperous animals which lay eggs in water and all viviperous animals. e.g., – All viviperous animals (Mammals) and all oviperous animals which lay eggs in water (Amphibians).

Classification of egg – membranes

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

(1) Primary egg membrane : This membrane is secreted by the egg (ovum) itself. e.g., – Vitelline membrane of human egg.

(2) Secondary egg membrane : This is found outside the primary egg membrane and is secreted by the ovary. e.g., Chorion of insect eggs, corona radiata and zona pellucida of human egg.

(3) Tertiary egg membrane : This present outside the primary and the secondary egg membrane. It is either secreted by the uterus or the oviduct. Egg jelly coat around frog’s egg; albumen, shell membrane and shell of bird egg.

Functions of egg membranes

(1) To provide protection.

(2) To check polyspermy.

(3) To provide buoyancy to the amphinian eggs.

(4) To provide nutrition (Birds, Reptiles)

(5) To help in excretion (Allantois)

Different types of eggs

(1) Insect egg : Eggs of insects are megalecithal or polylecithal in them yolk is present in the centre, so the eggs are also centrolecithal. Eggs of insects are cigar like. Along with plasma-membrane the egg has 2 egg-membranes.

(i) Vitelline membrane : This is a primary egg membrane and the egg itself secretes it around.

(ii) Chorion : This is a secondary egg membrane and is secreted by the ovary. In Insect’s egg tertiary egg-membranes is absent. Chorion of insect’s egg is ornamented i.e. there are specific markings on its egg membrane which are characters of Taxonomic importance. In the egg, a hole termed as micropyle is present which is the port of entry for sperms. Its cytoplasm is divided into 2 parts –

(a) Central cytoplasm

(b) Peripheral cytoplasm

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

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

Yolk : In insect’s egg yolk is present in a very large amount and this yolk is concentrated between the central and the Peripheral cytoplasm.

(2) Frog’s egg : Eggs of frog are Telolecithal and Mesolecithal. The egg has 2 egg membranes.

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

(ii) Jelly – coat : This is a tertiary egg-membrane. It is secreted by the oviduct. Secondary egg-membrane are absent in these egg’s. Internally, the egg is divided into 2 areas –

(a) Animal pole                                                   (b) Vegetal pole

(a) Animal pole : This part has more amount of cytoplasm in it and the egg nucleus is also located in it. In this part melanin granules are found which prevent the egg from harmful radiations. Due to these melanin granules the frog’s egg is partly white and partly black. This helps in Camouflage. Sperm always enters inside the egg through the animal pole. The part from where the sperm enters inside the frog’s egg in future forms the ventral part of the embryo. As the sperm enters inside the egg. The part directly opposite to the entry point becomes a clear-zone due to the rapid movement of melanin granules. this clear-zone is termed as the Grey-Crescent. This part with Grey-Crescent forms the dorsal part of the embryo in future.

(b) Vegetal pole : Here the yolk is concentrated in frog’s egg, the part with cytoplasm in future forms the ectoderm. The Grey crescent part in future the Mesoderm and the part with yolk in future forms the endoderm.

Jelly-coats of all the eggs of a frog absorb water and swell up, to form a cluster of eggs termed as Spawn. Jelly-coat has air-bubbles, due to which the eggs don’t drown. Jelly-coat is bitter in taste and so the eggs are protected from the enemies.

Phases of embryonic development

Embryonic development involves following dynamic changes and identifiable process.

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

(i) Spermatogenesis : Formation of sperm.

(ii) Oogenesis : Formation of ova

(2) Fertilization : It involve the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis.

(3) Cleavage : It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation.

(4) Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.

(5) Gastrulation : It includes the mass and orderly migration of the organ specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers.

(6) Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.

Fertilization

Definition : Fusion of a haploid male gamete (spermatozoan) and a haploid female gamete (ovum) to form a diploid cell, the zygote, is called fertilization or syngamy.

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

Steps of fertilization

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

Capacitation is the phenomenon of physiological maturation of sperms by breaking of acrosome membrane inside the female genital tract. It takes about 5-6 hours.

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

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