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[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.
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 Dictionary, © Random House, Inc. 2018.
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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
allantoic (ˌælənˈtəʊɪk) adjective
Word Origin
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
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allantois in Medicine

allantois al·lan·to·is (ə-lān'tō-ĭs)
n. pl. al·lan·to·i·des (āl'ən-tō'ĭ-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. Also called allantoid.

al'lan·to'ic (āl'ən-tō'ĭk) adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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allantois in Science
Plural allantoides (āl'ən-tō'ĭ-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 © 2002. Published by Houghton Mifflin. All rights reserved.
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