Chorion

Chorion (Gk. chorion, skin), one of the foetal membranes which plays an important part in development. The true chorion is described as including the vitelline membrane, the outer layer of the amnion, and the allantois. Its external surface is covered with processes, the chorionic villi, in which are formed loops of capillary blood-vessels. These capillaries undergo special development at the placental site, and, together with outgrowths from the maternal tissue, they gradually build up the highly vascular organ which forms the connecting link between the circulatory systems of mother and fcetus.