William Wood Russell (1899) and at about the same time the German authors von Franqué (1898), Naumann (1898), Agnes von Babo (1900) and Vassmer (1901) reported findings of endometrial tissue in the ovary. The organ in these cases was only slightly enlarged, and contained strands and ramifications of a tissue which showed in all its details the structure of the physiologic mucous membrane of the uterine body. That this pathologic condition is also of definite clinical importance was first demonstrated by Pick in 1905,1 who reported four cases from the operative material of Landau's clinic (Berlin). In the second and fourth cases, there were bilateral or unilateral ovarian cysts ranging from the size of an apple to that of a goose egg, and the operation in these cases consisted of oophorectomy solely for this condition. The tissue of the lining of the cysts was found to be identical with