allantois

al·lan·to·is

noun \ə-ˈlant-ə-wəs\

plural al·lan·to·ides \ˌal-ən-ˈtō-ə-ˌdēz, ˌal-ˌan-\

Definition of ALLANTOIS

: a vascular fetal membrane of reptiles, birds, or mammals that is formed as a pouch from the hindgut and that in placental mammals is intimately associated with the chorion in formation of the placenta