Fertilization in sea urchin.
<P>
<Li> The acrosome releases enzymes to digest the jelly coat.
<Li> Actin filaments bind to receptors in the vitelline layer.
<Li> The  sperm  and  egg plasma membranes fuse and become depolarized,
preventing  polyspermy  -  the ability of other sperm to fertilize
the egg.
<Li> The sperm nucleus enters and fuses with the egg's nucleus.
<Li> The  vitelline  layer swells to form a fertilization envelope that
also blocks polyspermy.