Cystadenoma of the ovary, Masson Trichrome. 

This cyst is an ingrowth on the ovary surface, the capsule tissue that normally surrounds the ovary is seen at right (light blue). As an adenoma it consists of epithelial cells derived from glandular ovarian tissue. The cavities of the glands fill with mucous produced by the epithelial cells. 

The other day, a coworker grossed one that was a 5kg beast. Cutting into it was interesting to say the least.