chorion
1. The outer cellular layer of the embryo sac of reptiles, birds, and mammals. It consists of two epithelial layers, an ectoderm surrounding a mesoderm layer. In mammals, the outer epithelium (trophoblast) of the chorion forms the placenta, maintaining close contact with the maternal tissues. See also AMNION.

2. The structurally complex outer coat of an insect egg, which is often externally patterned, frequently reflecting the pattern of follicle cells in the ovary which laid it down.

chorion
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.

chorion (kor-iŏn) n. the embryonic membrane that totally surrounds the embryo from the time of implantation.
—chorionic adj.

chorionic villi pl. n. see villus.