Each ovary contains hundreds of thousands of follicles, each of which contains an oocyte.  An oocyte is a cell that has begun the process of meiosis and can eventually produce an ovum, or egg.  The follicles produce female sex hormones.  This diagram illustrates the stages a single follicle goes through.  Changes in the ovary are the result of changing levels of FSH and LH from the anterior pituitary.