Some articles on yolk sac, primary yolk sac:
... of the chorionic cavity (Extra-embryonic coelom) and the yolk sac (umbilical vesicle) is still up for debate ... The thought of how the yolk sac membranes are formed begins with an increase in production of hypoblast cells, succeeded by different patterns of migration. 8, the first portion of hypoblast cells begin their migration and make what is known as the primary yolk sac, or Heuser's membrane (exocoelomic membrane) ...
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