This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Germ cells appear in yolk sac at \nSelect and explain. \nA. 9 weeks, B. 6 weeks, C. 3 weeks, D. 5 weeks', 'role': 'user'}, {'content': 'Answer: C \nExplanation: By 11th\xa0to 12th-day cells appear between the inner surface of the cytotrophoblast and the outer surface of the yolk sac. \nThese cells lie loosely and are called extraembryonic mesoderm. A cavity formed within it called the extraembryonic coelom or chorionic cavity. \nThis covers both the yolk sac and amniotic cavity except where the embryoblast is connected to trophoblast through the connecting stalk', 'role': 'assistant'}]