[uh-lan-toh-is, -tois]
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.

1640–50; < Neo-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. 2014.
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World English Dictionary
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
[C17: New Latin, irregularly from Greek allantoeidēs sausage-shaped, allantoid]

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American Heritage
Medical Dictionary

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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American Heritage
Science Dictionary
allantois   (ə-lān'tō-ĭs)  Pronunciation Key 
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.
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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica


an extra-embryonic membrane of reptiles, birds, and mammals arising as a pouch, or sac, from the hindgut. In reptiles and birds it expands greatly between two other membranes, the amnion and chorion, to serve as a temporary respiratory organ while its cavity stores fetal excretions. In mammals other than marsupials the allantois is intimately associated with the chorion, contributing blood vessels to that structure as it forms-in conjunction with the endometrium, or mucosal lining, of the uterus-the placenta

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
In reptiles, birds, and many mammals the allantois becomes expanded into a vesicle which projects into the extra-embryonic celom.
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