1. A membrane enclosing the embryo, yolk sac, and allantois of reptiles, birds, and mammals. In mammals a section of the chorion becomes the embryonic part of the placenta. See extraembryonic membranes.

2. The protective shell of an insect egg, produced by the ovary. It is pierced by a small pore (micropyle) that allows the entry of spermatozoa for fertilization. See also egg membrane.