Breadcrumb Home > Dictionary Cite Meaning of allantois Find definitions for: 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. See also: allantois (Thesaurus) allantoin alla prima Related Content Daily Word Quiz: apotheosis Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language