allantois defined in 1951 yearallantois - allantois;
allantois - Sac-like outgrowth of ventral side of hinder part of gut present in embryos of amniote vertebrates; represents a large and precocious development of urinary bladder. Allantois grows during development so that it extends right outside the embryo proper, to lie in wall of yolk-sac of birds and reptiles, or under chorion of mammals. It is always covered with connective tissue containing a rich network of blood-vessels, communicating with embryonic circulation. In reptiles and birds, respiration takes place via these blood-vessels, which lie immediately under outer layer of yolk-sac which itself is pressed close against inside of shell; excretory products are stored in allantoic cavity (See also: Uricotelic); and the greater part of allantois is left behind in the shell at hatching. In placental mammals the allantoic blood-vessels supply blood to placenta serving not only for respiration but for nutrition and excretion; cavity may be large and accumulate urine, but is often very small; most of allantois and its blood-vessels are detached from embryo at birth.
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