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allantois In tetrapod embryos, a sac formed by the outgrowth of the posterior ventral part of the gut, as a precocious urinary bladder. In reptiles and birds it grows to surround the embryo, lying between the yolk sac and shell; the blood vessels by which it is linked to the embryo provide the means of respiration, and the allantoic cavity receives metabolic wastes. Most of the allantois is left with the shell at hatching. In Eutheria the allantois forms part of the placenta, supplying it with blood for respiration, nutrition, and excretion, and the allantoic cavity may be small. Most of the allantois is detached from the embryo at birth.

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allantois One of the membranes that develops in embryonic reptiles, birds, and mammals as a growth from the hindgut. It acts as a urinary bladder for the storage of waste excretory products in the egg (in reptiles and birds) and as a means of providing the embryo with oxygen (in reptiles, birds, and mammals) and food (in mammals; see placenta). See also extraembryonic membranes.

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allantois (al-ăn-toh-iss) 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.
allantoic adj.