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
Extra-embryonic mesoderm is derived from: 
Select and explain. 
A. Epiblast, B. Primary yolk sac, C. Secondary yolk sac, D. Hypoblast
B: Primary yolk sac
Answer: B: Primary yolk sac 
Explanation: Ans: B. Primary yolk sac(Ref Gray's. 41/e p169, 40/e p167, 173, 186-189; Langmuir S 13/e p45,'Pe p54-55)Extra-embryonic mesoderm is derived from primary yolk sac.Formed by delamination of yolk sae cells - later by migration of cells through primitive streak during gastrulation.