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
Gastrulation occurs during which week after feilization? 
Select and explain. 
A. 1st, B. 2nd, C. 3rd, D. None of the above
C: 3rd
Answer: C: 3rd 
Explanation: The most characteristic event occurring during the third week is gastrulation, which begins with the appearance of the primitive streak, which has at its cephalic end the primitive node. In the region of the node and streak, epiblast cells move inward (invaginate) to form new cell layers, endoderm and mesoderm. Ref: Langman's embryology 11th edition Chapter 5.