The marsupials have a yolk sac placenta (the initial stage in the development of the placenta in placental mammals. In placental mammals, the chorion and the allantois together form the fetal side of ...
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 ...