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
[{'content': 'First germ layer to be formed is: \nSelect and explain. \nA. Ectoderm, B. Endoderm, C. Mesoderm, D. All of them develop simultaneously', 'role': 'user'}, {'content': "Answer: B \nExplanation: 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.", 'role': 'assistant'}]