1.) During fertilization, when a sperm comes in contact with the zona pellucida layer of the ovum a cortical reaction takes place which leads to a modification of the zona pellucida that blocks polyspermy .
2.) The acrosomal enzymes released by the head of the sperm begin to dissolve and the actin filament comes into contact with the egg's plasma membrane causes the release of the calcium from the storage site in the egg and raises the intracellular calcium concentration .It trigers the fusion of cortical granules membrane with the egg plasma membrane liberating the contents of the granules to the extra cellular space and establishes a permanent barrier to the prevent polyspermy