[uh-lan-toh-is, -tois] 
| a vascular, extraembryonic membrane of birds, reptiles, and certain mammals that develops as a sac or diverticulum from the ventral wall of the hindgut. |
s, wrongly taken for pl. and given a sing., on the model of words like hērōís (sing.), hērōídes (pl.)
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.
| 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. |