FERTILIZATION: FIRST TWO EVENTS
In fertilization, four events take place as a sperm fuses with an ovum to produce a zygote. In some species, some of these events may occur simultaneously:

1. contact/recognition
2. movement into the egg
3. egg activation
4. fusion

A sea urchin sperm is activated when it reaches the jelly coat. Membranes enclosing the acrosome (cap covering the head of the sperm) fuse, and pores in the membrane expand. When calcium ions from seawater enter the acrosome, it grows and discharges proteolytic enzymes that digest through the coat. If sperm and egg are the same species, bindin, a species-specific protein, will recognize the bindin receptors on the vitelline envelope (an acellular covering of the eggs of certain animals).

Enzymes help the sperm bore through the vitelline envelope. The sperm and egg plasma membranes form a fertilization cone which contracts to draw the sperm into the egg.

Most species have at least one method to prevent fertilization of an egg by more than one sperm (polyspermy). In the fast block, the egg plasma membrane is depolarized and cannot fuse with more sperm. The slow block to polyspermy (cortical reaction) requires up to five minutes and generates a more complete block.