NOTES ON Fertilization in Frog

Fertilization is the fusion of sperm with egg resulting  in the fomiation of zygote. It is characterized by the following events.
l. Fertilization is external.
2. It is mrmospermy, i.e. only one sperm fuses with the egg
3. The fertilized egg rotates in such a way that the animal hemisphere goes above. .
4. The jelly coat swells and increases in thickness.
5. The second meiotic division is completed resulting in the release of the second polar body. .
6. The sperm enters the egg in the animal hemisphere at an angle of 40o from the centre of animal pole.
 7. I mmediately after the entry of the spermatozoon into the egg, the vitelline membrane becomes elevated. This membrane is now called fertilization membrane. The space be-tween this membrane and the surface of the egg is called perivitelline space filled with a fluid called perivitellifie fluid. In this fluid, the fertilized egg can rotate freely. The rotation of the Zgg is inevitable for the normal process of development. mmediately after fertilization, the black pigmented animal pole gets placed above and the yolk-laden vegetal pole below.
8. Before the release of egg into the water, the jelly coat remains thin. As the egg is released into the water, thejelly coat absorbs water and begins to swell until the thickness of
jelly becomes twice the diameter of the egg.
9. The second maturation division is completed immedialely after fertilization. As a result, the fertilized egg releases the second polar body.
10. The egg pronucleus and sperm pronucleus fuse together to form the zygotic nucleus. This process is called amphimixis
11 . The entry of sperm uses another important reaction  in the egg cytoplasm.The black pigments exhibit active movement. On one side, just below the equater a crescent  like area appears; it will be grey in colour. This area is called grey crescent. It appears opposite to the point ofsperm 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 a definite bilateral symmetry in the fertilized egg. .The unfertilized
egg is radially symmetrical.
12. The spemi penetrates the egg perpendicular to the cortex. After penetration, the sperm moves in the cortex per- pendicularly, 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) ofthe sperm and the egg-nucleus.