How does the fertilization membrane form in sea urchin?

In sea urchins, the fertilization membrane forms shortly after sperm entry into the egg. Upon fertilization, cortical granules within the egg release their contents into the perivitelline space, which causes a chemical reaction that hardens the egg's outer layer. This process, known as the cortical reaction, results in the formation of a protective fertilization membrane that prevents polyspermy and provides a barrier to additional sperm. The membrane ultimately helps ensure proper development of the embryo.