yolk-sag defined in 1951 yearyolk-sag - yolk-sag;
yolk-sag - Sac containing yolk which hangs from ventral surface of very yolky vertebrate embryos (elasmobranchs, teleosts, reptiles, birds). Has outer layer of ectoderm; inner layer, usually absorptive, of endoderm; with mesoderm containing coelom and blood-vessels between. The endoderm is in effect a diverticulum of the gut, the yolk- or fluid-filled space within it communicating with intestine of embryo (not in teleosts). In mammals the yolk-sac, quite empty of yolk, is only the endodermal sac, with its layer of mesoderm, homologous with that of the other vertebrates; the ectodermal layer with its mesoderm is called part of the chorion. The same terminology is often applied to the other amniotes. As absorption of yolk proceeds, yolk-sac is withdrawn until it merges into the embryo; but most of it is cut off from the embryo at birth in mammal.
near yolk-sag in Knolik
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