<prompt> Germ cells appear in yolk sac at :3 weeks6 weeks9 weeks5 weeks  <response>: As early as the third week of gestation, primordial germ cells appear in the endoderm of yolk sac, and these migrate along the dorsal mesentry to the urogenital ridge by the eighth week Reference: Shaws textbook of Gynaecology 15th edition page 27 