an enveloping vascular fetal membrane formed by the fusion of chorion and allantois in reptiles, birds, and most mammals: that of chicks is used as a culture medium for growing viruses
See chorioallantois in American Heritage Dictionary 4
noun
The highly vascular fetal membrane that consists of the fused chorion and allantois, found adjacent to the eggshell in reptiles and birds and constituting the placenta in most mammals.