The fertilization envelope forms as a result of the cortical reaction that follows sperm entry. Specifically, cortical granules beneath the egg plasma membrane fuse with the membrane and release their contents into the perivitelline space. This release causes structural changes in the outer layers of the egg, including modification and hardening of the vitelline envelope (the extracellular layer just outside the plasma membrane) and influx of water, which lifts the vitelline envelope away from the egg surface to become a thick, protective barrier—the fertilization envelope. This envelope acts to block additional sperm from reaching the egg, ensuring monospermy. In many species (notably sea urchins), enzymatic remodeling of the envelope and cross-linking events solidify the barrier, making it a durable, cross-linked protective coat around the fertilized egg.