Types of gametogenesis :

(i) Spermatogenesis
 The process of formation of sperms in the germinal-epithelium 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.
 Some special cells are present in this germinal epithelium which are called the 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 Subtentacular 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.
 When sperms fully mature, they move away from sertoli cells and get liberated in the cavity of seminiferous tubules.
 Liberation of sperms from sertoli cells is termed as Spermiation.
 Liberation of sperms from the testis is termed as Semination.
 Liberation of sperms into the vagina of the female is termed as Insemination.
 Sertoli cells are also endocrine in nature and they secrete 2 hormones �
AMH (Anti Mullarian Hormone) : This hormone stimulates degradation of female gonads in a male embryo.
Inhibine hormone : This hormone is secreted in adult stages and it stops the secretion or FSH.
 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 �

(a) Formation of spermatid.
(b) Spermiogenesis or Spermatolesis.

(a) Formation of spermatids : This process begins as the animal attains sexual maturity. The cells of the germinal epithelium of the seminiferous tubules 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 �
(1) 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.
(2) 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.
(3) 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.

(b) 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 �
(i) From nucleus and glogibody -> Head part
(ii) From mitochondria -> Middle part
(iii) From distal centriole -> Tail part.
(i) The structure of the head of the sperm mainly depends on the structure of the nucleus. During spermatoliosis, nucleus contracts and acquires different shapes.

 RNA and nucleolus disappear from nucleus or their major part is given out from the nucleus. DNA also contracts / shrinks i.e. now the nucleus contains only those materials which are responsible for the hereditery characters.
 Centrosome divides into 2 centrioles i.e. 1 distal and 1 proximal centriole.
 Many golgi-bodies aggregate on the posterior side and their vacuoles enlarge.
 In some vacuoles of the golgi-bodies. Some dense-bodies can be seen, which are termed as Acroblast.
 In any one vacuole the Acroblast enlarges by aggregating with other. These are termed as �Acrosomal Granules�.
 The rest part of the golgi-body is given out and this is termed as Golgi-rest.
 This acroblast reaches the anterior most tip of the sperm and acquires a cap shaped structure on the nucleus which is termed as the Acrosome.
 Acrosome is surrounded by a double membrane i.e. the tonoplast and cell-membrane.
 The acrosome along with this membrane is termed as Galea-capatis. Acrosome, at the tip of the nucleus has an important role in breaking the egg.
 Mitochondria from different parts of spermatid, collect around the growing axonema and form a dense structure. In mammals, they form a spiral structure called Nebenkern sheath. In other animals, they collect and form mitochondrial clumps.
 The part of the axonema surrounded by mitochondria is known as Middle-piece.
 Around the middle-piece cytoplasm is present in the form of a thin layer called Manchette.
 Behind the middle piece, axonema surrounded only by plasmalemma is present. This is called the tail part.
 Shape of the sperm is so that it can move easily in a liquid medium.
 Structures and functions of sperms parts �
(a) Acrosome : Breaking the egg
(b) Middle piece : For providing energy
(c) Tail : For locomotion
Above structures are formed during spermatoliosis and the remaining substances are given out in the form of cytoplasmic residue, which is ingested by the "Cells of Sertoli".

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, certain cells in the germinal epithelium of the ovary are larger than others and also have larger nuclei.
 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
 Meosis occurs. Nucleus shifts towards animal pole and undergoes meosis � 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 meosis � II but stops at metaphase-II. Ovum is generally shed in secondary oocyte stage.
 After fertilization, the second meotic 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 meotic 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 three haploid polar bodies. The ootid grows into a functional haploid ovum.
 The polar bodies have no function and disintegrate due to lack of cytoplasm and food.
 The formation of non functional polar bodies enables the egg to get rid of excess chromosomes. The unequal cytoplasmic division enables the ovum to retain the whole of cytoplasm of the primary oocyte in it for the development of the future embryo.
 In humans, ova are released from the ovary in the secondary oocyte stage. Their maturation is completed in the mother's genital tract, usually after the sperm has entered for fertilization.

(iv) 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 10 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 50micron in many polychaete worms, 150micron in tunicates but very large sized in birds and reptiles. In mammals, it is generally microlecithal and about 100micron.
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 cortex is with some cytoskeletal structures like microtubules and microfilaments (Balinsky, 1981), pigment granules and cortical granules of mucopolysaccharides. Endoplasm is with cell-organelles, informosomes tRNAs, histones, enzymes etc. Nucleus of ovum is large, bloated with nucleoplasm and is called germinal vesicle. Nucleus is excentric in position so human ovum has a polarity. The side of ovum with nucleus and polar body is called animal pole, while the opposite side is called vegetal pole.

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.