An Introduction to Zoology: Chapter 6

The Chorion:

The chorion is the outermost of the four extraembryonic membranes, and its primary role is in gas exchange. You can easily find the chorion in a chicken egg; it’s the thin membrane that lies just under the shell.

In amniotes that lay eggs, oxygen diffuses into the egg across the amnion as carbon dioxide diffuses across it in the opposite direction and to the outside. As the embryo inside the egg grows and its oxygen demands increase, the chorion partially fuses with the allantois to form the chorioallantoic membrane. The chorioallantoic membrane has a rich supply of blood vessels, allowing for rapid exchange of oxygen and CO2 between the embryo and the outside environment.

In placental mammals, extensions of the chorion known as chorionic villi penetrate into the lining of the mother’s uterus. This brings embryonic and maternal blood vessels into close proximity, allowing for efficient exchange of nutrients and gases between the mother and the developing embryo.