1. Post meiosis I egg + preacrosomally reacted sperm collide.
  2. Sperm undergoes acrosome reaction on binding to zona pellucida or expanded cumulus glycoproteins.
  3. Sperm penetrates zona via the lytic enzymes of the acrosome and gets to perivitelline space.
  4. Binds to specific receptors on the egg.
  5. Egg responds by producing a Ca++ wave.
  6. Egg then resumes meiosis II and extrudes (loses) the 2nd polar body.
  7. Coincidently, the egg engulfs the sperm.
  8. Also coincidently, granules in the egg cytoplasm cortex (near the egg surface) exocytose and release enzymes that alter the zona pellucida and the egg surface so other sperm can't bind: cortical reaction = block to polyspermy.