This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Embryology 2, Subject name: Anatomy
Where do primordial germ cells initially develop 
Select and explain. 
A. In the gonads at week 4 of embryonic development, B. In the yolk sac at week 4 of embryonic development, C. In the gonads at month 5 of embryonic development, D. In the yolk sac at month 5 of embryonic development
B: In the yolk sac at week 4 of embryonic development
Answer: B: In the yolk sac at week 4 of embryonic development 
Explanation: Primordial germ cells, the predecessors to gametes, are first seen in the wall of the yolk sac at week 4 of embryonic development They migrate into the gonads at week 6.