Allantois

The allantois (/əˈlæntoʊɪs/ a-LAN-toe-iss; pl.: allantoides or allantoises) is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo exchange gases and handle liquid waste.

The allantois, along with the amnion, chorion, and yolk sac (other extraembryonic membranes), identify humans and other mammals, birds, and reptiles as amniotes. These extraembryonic membranes that form the embryo have aided amniotes in the transition from aquatic to terrestrial environments. Fish and amphibians are anamniotes, and lack the allantois. In mammals the extraembryonic membranes are known as the fetal membranes.