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
The notochord formation occurs on approximately what day after conception? 
Select and explain. 
A. Day 2, B. Day 7, C. Day 17, D. Day 28
C: Day 17
Answer: C: Day 17 
Explanation: Formation of the notochord is a third week event, occurring as a relatively early pa of development of the trilaminar disc. More specifically, notochord formation begins about day 16 and the neurenteric canal forms on about day 18. Two days after conception, implantation has not yet occurred. By seven days after conception, the blastocyst has usually implanted in the endometrium. By day 28, gastrulation has occurred, and organogenesis is under way.