This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
[{'content': 'Primordial germ cells are seen in ------ by 3rd week of development? \nSelect and explain. \nA. Neural crest, B. Splanchnic mesoderm, C. Genital ridge, D. Yolk sac', 'role': 'user'}, {'content': "Answer: D \nExplanation: Primordial germ cells originate in the epiblast, migrate through the primitive streak, and by the third week reside among endoderm cells in the wall of the yolk sac close to the allantois. Germ cells do not appear in the genital ridges until the sixth week of development Ref: Langman's embryology; Chapter 16; Pg no 243", 'role': 'assistant'}]