Ion responsible to prevent polyspermy at the time of feilization in mammals is -

Ans. is 'c' i.e., Ca Only one sperm can enter into oocyte, and fuher entry of sperms is prevented by the activation of ovum. Fusion of membranes of the gametes (sperm and ovum) leads to ovum activation which involves following events:- Membrane potential of ovum decreases (depolarization). Release of calcium ions from intracellular egg reserve leads to exocytosis of coical granules into the perivitelline space. The spread of coical granules along perivitelline membrane prevents fuher entry of sperm into the ovum. This is called vitelline block to polyspermy.