- A fetal membrane developing from the hindgut (or yolk sac, in humans). In humans it is vestigial; externally, in mammals, it contributes to the formation of the umbilical cord and placenta; in birds and reptiles, it lies close beneath the porous shell and serves as an organ of respiration. SYN: allantoid membrane. [allanto- + G. eidos, appearance]
* * *al·lan·to·is ə-'lant-ə-wəs n, pl al·lan·to·ides .al-ən-'tō-ə-.dēz, .al-.an- 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
* * *n.the membranous sac that develops as an outgrowth of the embryonic hindgut. Its outer (mesodermal) layer carries blood vessels to the placenta and so forms part of the umbilical cord. Its cavity is small and becomes reduced further in size during fetal development (see urachus).• allantoic adj.
* * *al·lan·to·is (ə-lanґto-is) [allanto- + eidos form] an initially tubular ventral diverticulum of the hindgut of embryos of reptiles, birds, and mammals. In reptiles and birds, it expands to a large sac for storing urine and, after fusing with the chorion which lines the shell, provides for gas exchange. The allantois is prominent in some mammals (carnivores, ungulates); in others such as humans, it is vestigial except that it becomes the urachus and its blood vessels give rise to those of the umbilical cord. allantoic adj
The small allantois of an early human embryo, shown in relation to the developing foregut, midgut, and hindgut.
Medical dictionary. 2011.