allantois

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

noun

Embryology, Zoology.

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

allantois

/ ˌælənˈtəʊɪk, əˈlæntɔɪs, ˌælənˈtəʊɪs /

noun

1. 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

allantois

plural

allantoides 

1. 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.

Other Word Forms

• allantoic adjective

Word History and Origins

Origin of allantois¹

1640–50; < New Latin < Greek allantoeidḗs, wrongly taken for plural and given a singular, on the model of words like hērōís (singular), hērōídes (plural)

Word History and Origins

Origin of allantois¹

C17: New Latin, irregularly from Greek allantoeidēs sausage-shaped, _allantoid

Example Sentences

The division of Vertebrata in which no allantois is developed.

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In Mammalia the allantois is not so largely developed as in Birds, and it enters largely into the formation of the placenta.

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The third is the chorion, forming the outer membrane which encloses the fetus, and within which is the amnion, or innermost membrane which surrounds the fetus, and between which is another membrane, called the allantois.

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Diagram of chick showing relations of amnion, allantois and yolk.

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The literal references to the foregoing are: am, amnion; al, allantois; v, vitellus; h, encephalon; i, eye; c, heart; f, liver; g, gizzard; ms, upper, and mi, lower member.

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