In this image of the mature cat ovary you can see two other stages of follicular development.  In primary follicles, the cells that surround the oocyte (developing egg) become cuboidal in shape and may be present one or several layers.  With increased growth and development of the follicle, the follicular cells form two and then several layers.  Later, a fluid-filled space (the antrum) begins to form within the follicular cells.  As soon as the follicle shows evidence of antrum formation it is called a secondary follicle.