Oogenesis is initiated during embryonic development stage and and couple of million gamete mor cells are formed within each fetal ovary. These cells start division and enter prophase 1 of meiosis division and get  temporarily arrested at that stage called primary oocytes. Furr, y get surrounded by a layer of granulosa cells and are called primary follicles. They gain new layer of granulosa cells and form secondary follicles which soon transform into tertiary follicles and contain a fluid-filled cavity called antrum. During this time primary oocyte completes its first meiotic division and results in formation of secondary oocyte. The tertiary follicle furr changes into mature follicle of Graafian follicle and forms a new membrane called zona pellucida.