Illustration of the stages of development of a human ova (egg) from a primordial follicle (top left) through to a graafian follicle (bottom left). Primordial follicles are produced in the ovary by the seventh month of foetal development, consisting of a primary oocyte surrounded by a layer of follicular cells. Futher growth is arrested until female reaches sexual maturity, when, under hormonal influences, the follicle develops through successive stages form a graafian follicle. During each ovarian cycle, up to 20 follicles develop; only 1 reaches full maturity to be released at ovulation (bottom right).