Is it ensure, insure, or assure?
chorion cho·ri·on (kôr'ē-ŏn')
The outer membrane enclosing the embryo in reptiles, birds, and mammals. In placental mammals it contributes to the development of the placenta.
in reptiles, birds, and mammals, the outermost membrane around the embryo. It develops from an outer fold on the surface of the yolk sac. In insects the chorion is the outer shell of the insect egg.