The fetal membranes consist of the chorion, amnion, yolk sac, and allantois. The chorion has three layers: cytotrophoblastic, chorionic connective tissue, and basement membrane. The chorion lies ...

A crucial evolutionary step saw the emergence of amniotic eggs. These eggs developed three extra membranes: the chorion, amnion, and allantois. Together, they formed a complete life support system ...