The histogenesis of cystic tumors of the ovary is not yet definitely settled. Every structure that enters into the composition of the ovary has been supposed to form the starting-point of these tumors. There are many classifications of ovarian cysts based upon the clinical, structural, or genetic features. The classification given here seems to me to be the best we have at present for the practical physician. [Illustration: FIG. 164.--Diagram representing the cyst-regions of the ovary and broad ligament.] Cystic tumors of the ovary may be divided into two general classes: I. Oöphoritic cysts, which originate from the oöphoron, or the egg-bearing portion of the ovary. II. Paroöphoritic cysts, which originate in the paroöphoron. OÖPHORITIC CYSTS. Cysts of the oöphoron may be subdivided into (_a_) Follicular cysts; (_b_) Glandular cysts; (_c_) Dermoid cysts. =Follicular Cysts.=--Follicular cysts originate in the ovarian follicles. If anything occurs to prevent the physiological rupture of a mature ovarian follicle, a follicular cyst may be started. Such cysts begin as retention-cysts of the ovarian follicles. The condition is usually the result of chronic inflammation. The formation of new connective tissue in the ovarian stroma, the thickening of the tunica albuginea, the presence of inflammatory exudate upon the surface of the ovary, may all prevent the rupture of the follicles. In addition, the inflammatory congestion of the walls of the follicle produces an increased exudation into the ovisac. [Illustration: FIG. 165.--Follicular cyst of the ovary.] It seems probable that such inflammatory action may also produce cystic distention in the immature follicles that are situated remote from the surface of the ovary. Follicular cysts may occur at any age, though they are most common during the period of sexual activity. The follicular cysts may occur in one or in both ovaries; usually both ovaries are affected. Only one follicle may be involved, or a large number of follicles, in different degrees of cystic distention, may be found scattered throughout the ovary. Frequently one follicle enlarged to the size of a hen’s egg is observed projecting from the surface of the ovary. Sometimes the intervening septa atrophy, and one large cavity is formed by the union of two or more cystic follicles.