This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
Which of the following is present at the beginning of 3rd week? 
Select and explain. 
A. Notochord, B. Primitive streak, C. Mesoderm, D. Neural crest cells
B: Primitive streak
Answer: B: Primitive streak 
Explanation: Primitive streak appears at the end of the 2nd week of intrauterine life and beginning of 3rd week, epiblast cells in the primitive streak forms the notochord, mesoderm and neural crest cells later in the third week. Disc with three germ layers are formed at 3rd week of gestation. Ref: Gray's 39e/p-210-220