Allantois

al;lan;to;is pronunciation: function: noun inflected form(s): plural al;lan;to;ides etymology: new latin, ultimately from greek allant-, allas sausagedate: 1646 : a vascular fetal membrane of reptiles, birds, and 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— al;lan;to;ic adjective