THE affections of the reticulo-endothelial system have long been the object of most lively interest on the part of the pathologists. In recent years, the malignant tumors arising from the reticuloendothelial tissues, in particular, have attracted the attention also of the radiologists, and it therefore seems to me a suitable subject to bring before this Congress, especially as there are several questions in this connection that are in need of further discussion. Pathology Already, in 1914, Ewing had discerned a species of tumors emanating from the reticulum cells of the lymphatic tissue, and in his text-book he definitely speaks of reticulum-cell sarcoma as different from the lymphocytoma. In 1930, Roulet, of Berlin, described in detail, under the name of retotheliosarcoma, a group of tumors arising from the lymphatic tissue in lymph nodes, the epipharynx, the tonsils, and other parts of the upper air passages—growths which are doubtless identical with the above-named reticulum-cell sarcomas of Ewing, but...