chorio-, chori-

Extraembryonic membrane formed in birds and reptiles by the apposition of the allantois to the inner face of the chorion.

The chorioallantoic membrane is highly vascularized and is used experimentally as a site upon which to place pieces of tissue in order to test their invasive capacity.

1. The highly vascular fetal membrane that consists of the fused chorion and allantois, found adjacent to the eggshell in reptiles and birds and constituting the placenta in most mammals.
2. A membrane surrounding an embryo.

In a bird's or reptile's egg, it lies next to the shell. In mammals, it forms a major part of the placenta.

1. The outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds and mammals).
2. The outermost of the two fetal membranes, the amnion being the innermost, which together surround the embryo.

The chorion develops villi (vascular fingers) and gives rise to the placenta.

In Greek, the word chorion means, "skin" or "leather".