Ovarian follicle. Coloured scanning electron micrograph (SEM) of a fracture through a secondary follicle in the ovary. The oocyte (developing egg) is orange and its central nucleus is darker orange. Surrounding the oocyte is the zona pellucida (green) and a layer of follicular cells known as the corona radiata (purple). Magnification: x1350 when printed at 10 centimetres wide.