Yolk sac

The yolk sac is the exclusive organ of nutrition of the embryos of oviparous vertebrates. Viviparous sharks, reptiles and mammals also usually form from placentas that ensure the course of further embryonic development more or less the complete nutrition on the maternal circulation. In the yolk- poor eggs of the higher mammals, the yolk sac is not only a phylogenetic relic ( rudiment ), but a necessary element in the early phase. In humans, it reaches a size of up to 5 mm and accepts prior to the formation of the liver in part their metabolic function. From the wall of the yolk sac, the germ cells and stem cells for blood formation are shown. With the pinch of the intestinal tube from the yolk sac and the lateral folding of the embryo, the small yolk sac lies in the chorionic cavity and communicates with the intestine via the yolk passage. The intestinal tube is divided out from the roof of the yolk sac. In some mammals (eg, horses, dogs, cats) is temporarily a real yolk sac placenta and the yolk sac remains until the birth as a small bag.

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