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allantois

Syllabification: (al·lan·to·is)

Pronunciation: /əˈlantō-is/

Definition of allantois

noun (plural allantoides /ˌalənˈtō-idēz/)

the fetal membrane lying below the chorion in many vertebrates, formed as an outgrowth of the embryo’s gut. In birds and reptiles it grows to surround the embryo; in eutherian mammals it forms part of the placenta.