The cortical reaction is a process initiated during fertilization that prevents polyspermy, the fusion of multiple sperm with one egg.

This releases the contents of the cortical granules outside the cell, where they modify an existing extracellular matrix to make it impenetrable to sperm entry.

In most animals, the extracellular matrix present around the egg is the vitelline envelope which becomes the fertilization membrane following the cortical reaction.

Although highly conserved across the animal kingdom, the cortical reaction shows great diversity between species.

The released cortical granule proteins exert a colloid osmotic pressure causing water to enter the space between the plasma membrane and the vitelline layer, and the vitelline layer expands away from the egg surface.