in mammalian fertilization, the cortical reaction modifies zona pellucida as a slow block to polyspermy bilization is followe cleavage, a period of rapid division without growth producing a large number cells called blastomeres. The atnount and distribution of yolk strongly influence the pattern of cleavage. The midblastula transition marks the end of cleavage, which many species generates a blastula containing a fluid cavity, the blastocoel.