The zona pellucida is a membrane composed of four glycoproteins (ZP1-4) that surrounds the plasma membrane of the mammalian oocyte. It is important for inducing the acrosome reaction during fertilization and preventing polyspermy. The zona pellucida persists through pre-implantation embryo development, but the blastocyst must hatch out of the zona pellucida in order to undergo implantation in the uterus.