chorion /kôr″ē·on/ [Gk, chorion, skin] , the outermost extraembryonic membrane composed of trophoblast lined with mesoderm. It develops villi about 2 weeks after fertilization and is vascularized by allantoic vessels 1 week later. It gives rise to the placenta and persists until birth as the outer of the two layers of membrane containing the amniotic fluid and the fetus. Compare amnion. See also amniotic sac.
