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: Anatomy
Yolk sac is derived from 
Select and explain. 
A. Hypoblasts, B. Epiblasts, C. Syncytiotrophoblast, D. None
A: Hypoblasts
Answer: A: Hypoblasts 
Explanation: Ans. is 'a' i.e., HypoblastsThe yolk sac develops from hypoblast cells. By the end of 2nd week the lower half of primary yalk sac pinches off to form definitive yolk sac, while a second wave of hypoblast cells form inner lining of definitive yolk sac.