In placental mammals, its nutritional function is vestigial however, it is the source of intestinal mucosa blood cells and germ cells. In reptiles and birds, it arises from endoderm and mesoderm to incorporate the egg yolk into the digestive tract for nourishing the embryo. The process starts with the migration of primordial germ cells from the yolk sac Yolk Sac The first of four extra-embryonic membranes to form during embryogenesis. In females, oogenesis results in an ovum. In males, spermatogenesis produces spermatozoa. Gametogenesis is the development of gametes from primordial germ cells. Students: Educators’ Pro Tips for Tough Topics.Maternity Nursing and Care of the Childbearing Family.
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