This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Germ cells appear in yolk sac at : \nSelect and explain. \nA. 3 weeks, B. 6 weeks, C. 9 weeks, D. 5 weeks', 'role': 'user'}, {'content': "Answer: A \nExplanation: 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: Shaw's textbook of Gynaecology 15th edition page 27", 'role': 'assistant'}]