Write short notes on following events of the process of fertilisation:
(i) Penetration of sperm into ovum. (ii) Cortical reaction of the ovum.

Explanation

(i) Penetration of sperm into ovum is a chemical phenomenon. When the sperm comes in contact with ovum, the acrosome of sperm secretes sperm lysins which dissolve the egg envolopes locally and make the path for sperm penetration, Sperm lysins include hyaluronidase (which dissolves hyaluronic acid binding the follicular cells of corona radiata), corona penetrating enzyme and acrosin (which dissolves the zona pellucida). (ii) Cortical reaction of the ovum. It is the reaction of cortical granules of the ovum after the penetration of sperm into Ovum. It involves lifting of vitelline membrane, formation of a perivitelline space below the vitelline membrane, extrusion of cortical granules in perivitelline space by exocytosis and thickening of vitelline membrane into a fertilization membrane to prevent the polyspermy.