Ovum development. Illustration showing the development of a human ovum (egg) in an ovary (pink, at lower centre) during the menstrual cycle. While up to twenty egg-producing primary follicles (small circles in left of ovary) begin to develop, only one reaches maturity. This follicle can be seen developing anti-clockwise from lower centre. It forms a secondary follicle and then a Graafian follicle (largest stage) prior to ovulation, when the ripe ovum is released into the fallopian tube (dark pink). If the ovum is not fertilized the uterus endometrium, or lining (shown in background), is shed in menstruation and another follicle starts to develop.