The question of the relation of the spleen to the =fresh formation of red blood corpuscles= is a problem of comparative anatomy. Observations on this point made on one kind of animal can certainly not claim validity for other kinds. In lower vertebrates, as in fishes, frogs, tortoises, and also in birds, the blood-forming activity of the spleen is pronounced and of great importance. In mammalia on the other hand, in some cases this function cannot be demonstrated, and in others only to a very small degree. In the spleen of normal mice nucleated red blood corpuscles are seen in relatively large numbers; in the rabbit they are less numerous and often only to be found with difficulty. In the dog they only make their appearance after anaemia from loss of blood, normally they are absent. =In the human spleen nucleated red blood corpuscles are not to be found normally or in cases of severe anaemia, but exclusively in leukaemic diseases.= U. Gabbi in his recently published work on the haemolytic function of the spleen, also emphasises the difference between the various animal species. In guinea-pigs he found that the spleen acts largely as a scavenger of the red blood corpuscles; in rabbits very slightly. Consequently after removal of the spleen in guinea-pigs the number of red blood corpuscles rose 377,000 in the cubic millimetre, and the amount of haemoglobin 8.2%. After splenectomy in rabbits the increase in these values is absent. Shortly summarising our analysis of the facts before us, we must say that =the importance of the spleen for the production of the white blood corpuscles can in no respect be considerable=, and that if these cells really are produced by it, they must be free from granulations. The spleen therefore stands functionally in closer connection with the lymphatic gland system than with the bone-marrow. =The spleen has not the least connection with ordinary leucocytosis[14].= ([beta]) The Lymphatic Glands. As it is impossible experimentally to prevent the lymphatic glands as a whole from contributing to the formation of the blood, we are dependent almost entirely on clinical and anatomical researches for an elucidation of their function. Since Virchow's definition of the lymphocyte it has been admitted that the lymphocytes of the blood, both the small and larger kinds, are identical with those of the lymphatic glands and the rest of the lymphatic system. This is proved by the complete agreement in general morphological character, in staining properties of the protoplasm and nucleus, and from the absence of granulation.