Ovarian follicle. Light micrograph of a transverse section through a mammalian ovary, the reproductive structure that produces the female sex cells (ova). The section has revealed the internal structure of an ovarian follicle, the structure within which an ovum develops from an oocyte. This is a secondary ovarian follicle, which has outer follicle cells and follicular cavities, but doesn't contain a primary oocyte. Magnification: x195 when printed at 10 centimetres across.