[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.)
| 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. It is important in the formation of the umbilical cord and placenta in mammals. Also called allantoid. [New Latin, from allantoīdes, from Greek allantoeidēs, sausage-shaped : allas, allant-, sausage + -oeidēs, -oid.] al'lan·to'ic (āl'ən-tō'ĭk) adj. |
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
an extra-embryonic membrane of reptiles, birds, and mammals arising as a pouch, or sac, from the hindgut. In reptiles and birds it expands greatly between two other membranes, the amnion and chorion, to serve as a temporary respiratory organ while its cavity stores fetal excretions. In mammals other than marsupials the allantois is intimately associated with the chorion, contributing blood vessels to that structure as it forms-in conjunction with the endometrium, or mucosal lining, of the uterus-the placenta
Learn more about allantois with a free trial on Britannica.com.