This is a section through an enlarged 12 cm ovary to reveal 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 impart a brown to black color to the cyst contents.

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