Fertilization in sea urchin.

1. The acrosome releases enzymes to digest the jelly coat.

2. Actin filaments bind to receptors in the vitelline layer.

3. The sperm and egg plasma membranes fuse and become depolarized, preventing polyspermy - the ability of other sperm to fertilize the egg.

4. The sperm nucleus enters and fuses with the egg's nucleus.

5. The vitelline layer swells to form a fertilization envelope that also blocks polyspermy.