23. According to the chart below, when does the corpus luteum begin to degenerate (break down) if an egg does not implant in the uterine lining?
![](../pics/menstrualcycleqz1.gif)
(Click picture to see it full sized!)
A) About Day 5
B) About Day 13 or 14
C) About Day 22
D) About Day 28
Slichter