The thick, transparent, noncellular layer that surrounds a developing egg cell in the ovarian follicle. At fertilization, the zona pellucida is penetrated by at least 1 sperm.
the thick membrane that develops around the mammalian oocyte within the ovarian follicle. It is penetrated by at least one spermatozoon at fertilization and persists around the *blastocyst until it reaches the uterus. See ovum.
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