This image shows a section through the cortical region of a mature cat ovary.  The outermost layer consists of a very thin, darkly staining germinal epithelium, beneath which is a  thicker layer of fibrous tissue called the tunica albuginea.  The earliest stage of follicular development are the primordial follicles,  which consist of a primary oocyte surrounded by a single layer of flat follicular cells.  The cortex of the ovary contains many primordial follicles, which may eventually  develop into mature Graafian follicles.