allantois

noun

al·​lan·​to·​is ə-ˈlan-tə-wəs How to pronounce allantois (audio)
plural allantoides ˌa-lən-ˈtō-ə-ˌdēz How to pronounce allantois (audio)
ˌa-ˌlan-
: a vascular fetal membrane of reptiles, birds, and mammals that is formed as a pouch from the hindgut and that in placental mammals is intimately associated with the chorion in formation of the placenta
allantoic
ˌa-lən-ˈtō-ik How to pronounce allantois (audio)
ˌa-ˌlan-
adjective

Word History

Etymology

borrowed from New Latin, probably taken as singular of earlier allantoidēs in the same sense, borrowed from Greek allantoeidḗs "sausage-like" (in allantoeidḕs hymḗn "allantois," literally, "sausage-like membrane"), from allant-, allâs "sausage" (of uncertain origin) + -oeidḗs -oid entry 2

First Known Use

1615, in the meaning defined above

Time Traveler
The first known use of allantois was in 1615

Rhymes for allantois

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Browse Nearby Words

allantoinase
allantois
allantoxaidin
See all Nearby Words

Cite this Entry

“Allantois.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/allantois. Accessed 30 Oct. 2025.

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Medical Definition

allantois

noun
al·​lan·​to·​is ə-ˈlant-ə-wəs How to pronounce allantois (audio)
plural allantoides ˌal-ən-ˈtō-ə-ˌdēz How to pronounce allantois (audio) ˌal-ˌan- How to pronounce allantois (audio)
: a vascular fetal membrane of reptiles, birds, or mammals that is formed as a pouch from the hindgut and that in placental mammals is intimately associated with the chorion in formation of the placenta

More from Merriam-Webster on allantois

Britannica.com: Encyclopedia article about allantois

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