the zona pellucida has receptors for the sperm. The sperm has a structure on its head, which when it is matured, contains enzymes which can breakdown the corona radiata of the oocyte, the reaction in which these enzymes are released is called the acrosome reaction. Once one sperm fertilizes, a series of reactions occur but, mostly there is an increase in Ca++ concentration which hardens the zona pellucida and prevents any other sperm from entering to avoid polyspermia, which in most cases can be fatal (i think thats what its called) hope this helps.