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The entrance from the yolk sac into the primitive gut is ovoid in shape because the umbilical veins constrict the entrance laterally. The yolk sac is covered with a vascular plexus and blood islands that drain to the sinus venosus of the heart by the large umbilical and vitelline veins on each side. The primordial germ cells are located in the wall of the yolk sac near the allantois. They will later migrate in the hindgut mesentery to the dorsal body wall.


As the head and lateral body folds form, the cranial attachment of the amnion moves to the ventral surface of the embryo where the pericardial cavity meets the yolk sac. As the tail and lateral body folds form, the caudal attachment of the amnion moves to the ventral side where the connecting stalk attaches.


The connecting stalk lies between the amnion and yolk sac when the body folds develop. It contains a large, common umbilical artery and two umbilical veins that conduct blood to and from the chorion.


As the tail fold develops, the attachment of the allantois moves ventrally to form part of the ventral wall of the hindgut. It remains a diverticulum lined with endoderm and ends blindly in the connecting stalk.

Source: Atlas of Human Embryos.