chorion

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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.

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