Fertilization in mammals.
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<BR><LI> The sperm migrates through the coat of follicle cells.
<BR><LI> The acrosome releases enzymes to digest the zona pellucida.
<BR><LI> The  plasma  membranes of the two gametes fuse, allowing the whole
sperm to enter the egg.
<BR><LI> The egg releases enzymes from cortical granules to harden the zona
pellucida, which functions as a block to polyspermy.