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Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
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Germ cells appear in yolk sac at
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The correct answer is  By 11th to 12th-day cells appear between the inner surface of the cytotrophoblast and the outer surface of the yolk sac. 
These cells lie loosely and are called extraembryonic mesoderm. A cavity formed within it called the extraembryonic coelom or chorionic cavity. 
This covers both the yolk sac and amniotic cavity except where the embryoblast is connected to trophoblast through the connecting stalk