How is polyespermy prevented?

Explanation

To thin out the number of sperms, the ovum secretes a chemical substance called fertilizin(glycoprotein or mucopolysaccharide) which has a number of spermophilic sites on its surface where the sperms of species specific type can be bound by their antifertilizin site (on the surface of head of sperms and formed of acidic amino acids). This fertilizin-antifertilizin reaction is a highly species specific reaction (act as lock and key). The main aim of this reaction is to thin out the number of sperms to reduce the chances of polyspermy.