Figure 37.6: The Mammalian Ovary Showing Stages of Follicle Development The stages of follicle development are shown in a clockwise series, starting at the upper left with a primordial follicle. In a human ovary, a primordial follicle progresses through all these stages in the course of approximately 28 days (one menstrual cycle). A new primordial follicle starts the next month. The clockwise arrangement is for learning purposes only. Follicles do not actually progress clockwise through an ovary.