Fertilization Process
Frog Fertilization :
* Fertilization is the fusion of sperm with egg resulting in the formation of zygote.
Fusion of sperm and egg
* It is contains following steps :
* Fertilization is external.
* It is monopspermy i.e. only one sperm fuses with the egg.
* The fertilized egg rotates in such a way that the animal hemisphere goes above.
* The jelly coat swells and increase in thickness.
* The second meiotic division is completed resulting in the release of the second polar body.
* The sperm enters the egg in the animal hemisphere at n angle of 40 C from the center of animal pole.
* Immediately after the entry of the sperm into the egg, the vitelline membrane become elevated.
* The space between this membrane and the surface of the egg is called perivitelline space filled with fluid called perivitelline fluid.
* In this fluid, the fertilize egg can rotate freely.
* The rotation of the egg is inevitable for the normal process of development.
* Immediately after fertilization, the black pigmented animal pole gets placed above and the yolk-laden vegetal pole below.
* Before the release of egg into the water, the jelly coat remains thin.
* As the egg is released into the water, the jelly coat absorbs water and begins to swell until the thicken of the jelly becomes twice the diameter of the egg.
* The second maturation division is completed immediately after fertilization, as a result fertilized egg release the second polar body.
* The egg pronucleus and sperm pronucleus fuse together to from the zygote nucleus.
* This process is called as amphimixis.
* The entry of sperm causes another imp. Reaction in the egg cytoplasm.
* The black pigments exhibit active movement.
* On one side, just below the equator, a crescent like area appears; it will be grey in color.
* This area is called grey crescent.
* It appears opposite to the point of sperm entry.
* The region of the grey crescent will become the posterior side and the opposite region will become the anterior side of the future embryo.
* This leads to the formation of definite bilateral symmetry in the fertilized rgg.
* The unfertilized egg is radially symmetrical.
* The sperm penetrated the egg is the perpendicular to the cortex of the egg.
* After penetration, the sperm move in the cortex perpendicularly, along the radius of the egg.
* This path of the sperm is marked by pigment granules.
* This path of the sperm in the egg cortex is called penetration path.
* After crossing the cortex.
* The sperm changes its direction and moves towards the egg nucleus.
* This changed path is also marked by pigment granules and is called copulation path.
* Because it leads to the copulation (Fusion) of the sperm and the egg-nucleus.
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