Light microscopy of the ovary showing a recently ovulated follicle transforming into a corpus luteum. This developing corpus luteum ( yellow body') may be 5cm in diameter. It has a central core of fibrous material, blood and fluid which will disappear and diminish in size as the surrounding cells (wave-shaped tissue) originating from the follicle become luteal cells. Luteal cells secrete progesterone and also estrogen essential for maintenance of the uterine lining and implantation of the early embryo should the ovulated oocyte be fertilized. During pregnancy the corpus luteum survives for several months. If no pregnancy the corpus luteum degenerates and within ten days reverts to a contracted mass of fibrous tissue. Magnification x12 when printed at 10