The Chorion is the external covering of the ovum. At first it consists simply of the zona pellucida or vitelline membrane, and then it is called the primitive chorion. Later it is supplemented by that part of the somatopleure removed from the embryo in the process of forming the amnion. This blends with the primitive chorion and strengthens it, and while lying beneath the zona pellucida, receives the name of the subzonal membrane. The chorion at first is a smooth membrane, but villous processes early grow out from it. These villi are chiefly developed at its upper part, where they aid in the formation of the foetal placenta.

The allantois, when it has spread over the chorion, becomes blended with this membrane, and fills the villous processes with the blood vessels it contains.