[ uh-lan-toh-is, -tois ]
/ əˈlæn toʊ ɪs, -tɔɪs /

noun Embryology, Zoology.

a vascular, extraembryonic membrane of birds, reptiles, and certain mammals that develops as a sac or diverticulum from the ventral wall of the hindgut.



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Origin of allantois

1640–50; <New Latin <Greek allantoeidḗs, wrongly taken for plural and given a singular, on the model of words like hērōís (singular), hērōídes (plural) Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

Example sentences from the Web for allantois

British Dictionary definitions for allantois

/ (ˌælənˈtəʊɪs, əˈlæntɔɪs) /


a membranous sac growing out of the ventral surface of the hind gut of embryonic reptiles, birds, and mammals. It combines with the chorion to form the mammalian placenta

Derived forms of allantois

allantoic (ˌælənˈtəʊɪk), adjective

Word Origin for allantois

C17: New Latin, irregularly from Greek allantoeidēs sausage-shaped, allantoid
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for allantois

[ ə-lăntō-ĭs ]

n. pl. al•lan•to•i•des (ăl′ən-tōĭ-dēz′)

A membranous sac that develops from the posterior part of the alimentary canal in the embryos of mammals, birds, and reptiles, and is important in the formation of the umbilical cord and placenta in mammals.allantoid

Other words from allantois

al′lan•toic (ăl′ən-tōĭk) adj.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for allantois

[ ə-lăntō-ĭs ]

Plural allantoides (ăl′ən-tōĭ-dēz′)

A membranous sac that grows out of the lower end of the alimentary canal in embryos of reptiles, birds, and mammals. In mammals, the blood vessels of the allantois develop into the blood vessels of the umbilical cord.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.