This is a section through an enlarnged 12 cm ovary to demonstrate a cystic cavity filled with old blood typical for endometriosis with formation of an endometriotic, or "chocolate", cyst. The hemorrhage from endometriosis into the ovary may give rise to a large "chocolate cyst" so named because the old blood in the cystic space formed by the hemorrhage is broken down to produce much hemosiderin and a brown to black color.