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

/ ə-lăn′tō-ĭs /

, Plural allantoides ăl′ən-tō′ĭ-dēz′

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.

Derived 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 the embryo develops an allantois.

From Project Gutenberg

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

From Project Gutenberg

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.

From Project Gutenberg

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

From Project Gutenberg

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.

From Project Gutenberg