I. noun 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. II. derivatives 1. allantoic adjective
2. allantoid adjective
– origin mid 17th cent.: modern Latin, based on Greek allantoeidēs ‘sausage-shaped’.