Ovarian cycle, computer artwork. 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 (right) and release its egg at ovulation (bottom right). The follicle then develops into a corpus luteum (yellow), which secretes progesterone to build up the lining of the uterus, ready for a fertilised egg. If there is no fertilised egg after 12 days it stops secreting and degenerates into a corpus albicans (left).