Dictionary
al•lan•to•is
Pronunciation: (u-lan'tō-is, -tois), [key]—
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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.