بسم الله الرحمن الرحيم. Yolk Sac Origin: Primary yolk sac - Starts as vacuoles in the hypoblastic cells of the inner cell mass. - These vacuoles coalease.
Embryology 2010. History of Embryology Aristotle (384-322 BC) –The Generation of Animals (350 BC) »Oviparity »Viviparity »Ovoviviparity –Epigenesis (!)
