In the tertiary follicle, the primary oocyte completes meiosis I about 3 hours prior to ovulation to form the secondary oocyte and first polar body. These are located on one side of the follicle, surrounded by the zona pellucida and a layer of granulosa cells called the corona radiata. The oocyte also sits on a cushion of granulosa cells called the cumulus oophorous. A thick layer of granulosa cells lines the inside of the follicular space which is filled with a large accumulation of fluid known as liquor folliculi. The outer layer of the follicle wall is called the theca and it is divided into a theca interna and a theca externa. The tertiary follicle will continue to enlarge to become a graafian follicle. Eventually, following a surge of follicle stimulating hormone (FSH) form the anterior pituitary, the follicle will rupture and ovulation occurs.