Illustration of the uterus (right) and an ovary (centre left) on day twenty-one of the menstrual cycle. Once sexual maturity is reached up to 20 oocytes start developing into eggs (ovums) every menstrual cycle. Only one will reach full maturity as a Graafian follicle and release its egg (pink, in uterus) at ovulation. The follicle then develops into a corpus luteum (yellow, in ovary), which secretes progesterone to build up the endometrium, the bloody lining of the uterus (red), ready for a fertilised egg. If there is no fertilised egg after 12 days it stops secreting and degenerates into a corpus albicans. Without progesterone the endometrium sloughs off; this is menstruation. The cycle can now start again.