Endometriosis. Endoscope view of endometriosis on the peritoneum. The endometriosis cyst is the dark purple, small rounded area (centre). Endometriosis is a condition where detached parts of the inner lining (endometrium) of the uterus escape into the abdomen and attach themselves to internal organs. The lesions and cysts that result, bleed at the time of the menstrual cycle and cause great pain. The lesions and cysts can vary in appearance from red to white, clear and black. Treatment is usually with drugs. The peritoneum is the membrane that encases the abdominal cavity and its organs. A ligament (white, running from top left to lower right) anchors the uterus within the abdomen.