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
Primordial germ cell is derived from which of the following embronic layer: 
Select and explain. 
A. Ectoderm, B. Mesoderm, C. Endoderm, D. Mesodermal sinus
A: Ectoderm
Answer: A: Ectoderm 
Explanation: The primordial germ cells develop within the epiblast or the primary ectoderm and migrate to endoderm of the yolk sac wall.Ref: Gray's anatomy: the anatomical basis of clinical practice, Henry Gray, 39th Edition, Page 196, 1385; Fetal Medicine: Basic Science and Clinical Practice, Charles H, Main J. Whittle, 2nd Edition, Chapter 1, Page 4.