Fig. 9. Description of the progression of the “major wave” throughout successive cycles, from the entry of follicles into the preantral (class 1) to the preovulatory stage (class 8). Follicles that become preantral during the early luteal phase of every menstrual cycle have been endowed with selective advantages for further development. As growth is progressing, the initial number of follicles belonging to the major wave ( bold line) decreases by atresia, but in lesser proportion than do follicles from “minor waves” ( dotted line ). Consequently, when follicles belonging to major waves and of a given stage of their development ( e.g., class 1) enter the subsequent class ( e.g. class 2) during the late follicular phase, they are in larger numbers than at any other time during the cycle, explaining why this passage is morphologically discernible. (From Gougeon A: Regulation of ovarian follicular development in primates: Facts and hypotheses. Endocr Rev 17:121, 1996.)