KATERYNA KON / SCIENCE PHOTO LIBRARY KATERYNA KON / SCIENCE PHOTO LIBRARY
Ovarian cycle, computer illustration. 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 (lower left) and release its egg at ovulation (far left). 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 (green).
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