Cortical reaction with reference to fertilization
The entry of the sperm head into the egg brings about several changes in the egg surface. These are the cortical changes and the development of fertilization membrane. In some echinoderm eggs some fine granules are visible in the ooplasm after the entry of the sperm head.
The vitelline membrane starts lifting itself up from the point of sperm entry and a perivitelline space is formed between it and the egg surface. The cortical granules from the egg cortex release their contents into the privitelline space. These contents attach themselves to the inner surface of the vitelline membrane forming what is known as a fertilization membrane. The development of the fertilization membrane prevents the entry of other sperms into the egg.