The yolk sac connected to the gut by a narrow yolk stalk. The wall of the yolk sac is covered inside by the epithelium and outside by the connective tissue. The human yolk sac contains no yolk and has no nutritive function in man.
Function of yolk sac:
1. The first blood islands appear in mesoderm surrounding the wall of the yolk sac. It functions as primary hemopoiesis organ.
2. Yolk sac endoderm is origin for primordial germ cells, which then migrate to the primitive sex cords.
3. Roof of the yolk sac gives rise for the primitive gut,
Allantois
The caudal wall of the yolk sac forms a small diverticulum which extends into the connecting stalk. This diverticulum is called allantois and appears at about the 16th day of development. The proximal part of the allantois enters in the formation of the urinary bladder while its distal part (the urachus) is obliterated forming the median umbilical ligament which extends from the apex of the urinary bladder to the umbilicus. The allantoic vessels form the umbilical vessels. At early stages of embryogenesis allantois take place in the gas exchange and excretion. Also it has a role in the nutrition. During the second month, the extraembryonic part of the allantois degenerates.
Tissues
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