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Ovarian follicle. Coloured scanning electron micrograph (SEM) of an undeveloped human egg cell (primary oocyte, purple, centre) revealed by the fracturing of an ovarian follicle (large circle). An ovary contains many primary oocytes that do not develop further until hormonal signals trigger one to mature in each of a woman's menstrual cycles. The primary oocyte then develops into a secondary oocyte that carries half of the genetic material needed for reproduction. This secondary oocyte is released from the ovary and develops further as it forms an ovum during fertilisation, when the ovum fuses with the spermatozoan (male reproductive cell). Magnification: x80 when printed 10cm wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)