allantois

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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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