cho·ri·o·al·lan·to·is 
(kôr′ē-ō′ə-lăntō-ĭs)
(kôr′ē-ō′ə-lăntō-ĭs)n.
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.
cho′ri·o·al′lan·toic (-ō-ăl′ən-tōĭk) adj.