Gametogenesis,mating and fertilisation in frog

Spermatogenesis

The process of formation of sperm from the primary germ cell of male is called spermatogenesis.it occurs in seminiferous tubule of testis.It completes in following phases:

1. multiplication  phase : the germinal epithelium of testis undergoes mitotic division to produce many spermatogonia.spermatogonia is diploid cell.

2. growth phase : during growth phase,each spermatogonia accumulate nutrients and enlarges itself to form primary spermaocyte.

3.maturation phase:  each primary spermatocyte undergoes meiotic cell division to form secondary spermatocytes.secondary spermatocytes are haploid,they further undergoes meosis ii division to form spermatids. spermatids convert themselves into sperm.  sperms are haploid.

Structure of sperm :

Sperm is a microscopic,elongated gamete having haploid chromosome and measuring 0.03 mm.sperm mainly contains three parts. they are anterior broad head,slender middle piece and elongated flagella like tail.

Head consist of a distinct nucleus. head consist of acrosome in apical region.acrosome is a developed golgibodies and produce lysin to penetrate the egg membrane during fertilisation.

middle piece is slender and consists of mitochondria and centriole. mitochondria produce ATP that provides energy to sperm.

Tail is the long flagella like structure  of sperm which vibrates to bring the motility of sperm.

oogenesis

the process of formation of ovoum in female  from germinal epithelial cells is called oogenesis. it occurs in ovary of female.oogenesis occurs in following phases :

1.multiplication phase : the ovary of female is lined by germinal epithelium. the primary germ cell of germinal epithelium undergoes mitosis cell division for two  times and form oogonium.oogonium is diploid cell.

2.growth phase : during growth phase,each oogonium inlarges itself to form primary oocyte. inlargement takes place by accumulation of nutrients.

3. maturation phase : each primary oocyte further undergoes meosis division to form a large secondary oocyte(haploid) and a tiny polar body(haploid).

The polar body passes through meosis ii division and form two polarbodies.

The secondary oocyte(haploid) divides through meosis ii and form single ovum and a polar body.

Hence,at the end of maturation phase, 1 ovum and 3 polarbodies are formed. polar bodies soon degenerates and collapse.

Structure of ovum:

The yolk of frog is telolecithal i.e one  hemisphere is yolky and other is non yolky. egg of frog is spherical and have diameter 1.75mm.upper hemisphere is non yolky and is called animal pole.animal pole consist of cytoplasmic  contents and distinct nucleus.

lower hemisphere is non yolky and is called vegital pole.egg of frog  is covered by two membrane and they are plasmamembrane and vitelline membrane.when outer vitelline membrane comes in contavt with water it swells up to form jelly like structure and is protective in nature.

Mating :

Mating of frog takes place during rainy season. Male frog croaks to attract the female frog and grasps the bodt of female from behind. Male lets the  sperm go out in the water and this process is called insemination.

In the mean time, female frog lays egg in water and this process is called oviposition.female lays multiple egg at a time and form a group called as spawn.

Fertilization :

The process of fusion of haploid male gamete and haploid female gamete to form zygote is called fertilization. Fertilization is external in frog.

Frog  lays egg in the form of secondary oocyte in water. During the period of fertilization, numerous sperms gathers the  single egg. Soon a pore of reception is formed in animal pole region of egg from which only single sperm passes through.

The sperm enters the egg by loosing its tail. As soon as the sperm enters the egg, the vitelline membrane changes into fertilization membrane. Now the egg becomes activated and secondary oocyte changes into ovum and polar bodies.the ovum and sperm fuses to form zygote whereas polar bodies disintegrate.

The fertilization membrane checks the further entry of sperms. The side opposite to sperm soon forms a grey crescent which changes into dorsal body surface in future.