This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Development period- week 1,2,3,4, Subject name: Anatomy
The first indication of gastrulation in the embryo is 
Select and explain. 
A. Formation of the primitive streak, B. Formation of the notochord, C. Formation of the neural tube, D. Formation of extraembryonic mesoderm
A: Formation of the primitive streak
Answer: A: Formation of the primitive streak 
Explanation: The formation of the primitive streak on the dorsal surface of the bilaminar embryonic disk formed from proliferation at the end of 2nd week (day 14) is the first indication of gastrulation.