This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Embryology, Subject name: Anatomy
First germ layer to be formed is: 
Select and explain. 
A. Ectoderm, B. Endoderm, C. Mesoderm, D. All of them develop simultaneously
B: Endoderm
Answer: B: Endoderm 
Explanation: Ans. B EndodermRef: Netter's Atlas of Embryology, 2012 ed. pg. 47* Epiblast is the source of all three germ layers and endoderm is the first germ layer to be formed.