the membrane of a fetus that is formed by the merging of the allantois and the chorion
chorioallantois in American English
(ˌkɔriˌoʊəˈlæntoʊɪs)
noun
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