A 30-yeаr-оld wоmаn cоmes to the clinic due to pelvic pаin and inability to conceive during the past year. She stopped taking oral contraceptives a year and a half ago. The patient states that she often has severe pain with defecation and during sexual intercourse. Her menstrual cycle occurs every 28 days, lasts for 5 days, and is accompanied by severe pain and discomfort. Vitals are within normal limits. On physical examination, mild pelvic tenderness is noted, the uterus is soft and normal size. The cervix is normal and slightly immobile. A left adnexal area feels slightly enlarged. The patient's adnexal mass most likely derived from which of the following tissues?