The lymphoid structures may properly be considered as a part of the hematopoietic tissue of the body, since they contribute to the blood stream about 25 per cent of the white cells contained therein. Unlike the bone marrow, however, the lymphoid tissue has a local office to perform in addition to the general one of supplying cells to the circulation. This necessarily results in the separation of the diseases that affect the lymph nodes into two categories: those in which only a local reaction occurs and those in which the general lymphatic reaction is invoked. The latter group, often spoken of as the primary lymph node diseases, represents disorders, all of which are of unknown etiology and often of uncertain pathology, which initiate definite and often pathognomonic changes in the balance between the cells of the circulating blood. The former group, as might be anticipated, either fails to produce recognizable