Figure 27.4C is a cutaway view of an ovary. The series of follicles here represents the changes one follicle undergoes over time ; the arrows indicate the sequence. An actual ovary would have thousands of dormant follicles, each containing a primary oocyte. Usually, one follicle has a dividing oocyte at any one time, and as it develops, that follicle stays in one place in the ovary. Meiosis I occurs as the follicle matures. About the time the secondary oocyte forms, the pituitary hormone LH(luteinizing hormone ) triggers ovulation, the expulsion of the secondary oocyte from the follicle. The ruptured follicle then develops into a corpus luteum. Unless fertilization occurs, the corpus luteum degenerates before another follicle starts to develop.