/euh lan"toh is, -toys/ , n. Embryol., Zool.
a vascular, extraembryonic membrane of birds, reptiles, and certain mammals that develops as a sac or diverticulum from the ventral wall of the hindgut.
[ 1640-50; allantoeidés, wrongly taken for pl. and given a sing., on the model of words like heroís (sing.), heroídes (pl.) ]