It will be the task of further investigations to examine accurately inflammatory products, _e.g._ pleuritic exudations, in leukaemic patients, with the object of elucidating the question, whether under special conditions of disease all the leucocytes characteristic for leukaemia may not be able to wander from the blood. Thus in a case of pleurisy in a leukaemic patient, Ehrlich received the impression from the preparations that a "myeloid" emigration had in fact occurred, carrying all the elements in the blood into the exudation. This observation does not prove the point, for numerical estimation of the proportion of white to red blood corpuscles in the exudation was not made. And these estimations are necessary in order to prove indisputably the active emigration of the white blood corpuscles into the exudation, and to exclude their purely mechanical passage, _per rhexin_, from the blood-stream. The hypothesis of the active origin of myelaemia is considerably supported by a further train of argument. In leukaemia, besides the myelocytes, the polynuclear leucocytes are also enormously increased, and their active emigration is beyond doubt. And the view, that the mononuclear cells are washed into the blood, excludes that of a single mode of origin of the leukaemic blood condition; and commits us to a highly artificial explanation of its production. The morphological changes of leukaemic blood under the influence of infectious diseases can only be explained from the standpoint of the emigration theory. For if the white blood corpuscles were mechanically carried out of the bone-marrow as a whole, it is incomprehensible that a bacterial infection should alter this process to a polynuclear leucocytosis. This change of character is easily explained on the other hand, as we have above shewn more in detail, by the assumption that ordinary bacterial poisons act positively chemiotactically only on the polynuclear neutrophil cells, but negatively on the other forms. =We explain the origin of leukaemic blood by the emigration into the blood under the influence of the specific leukaemic agent, not only of the formed polynuclear elements, but also of their mononuclear, eosinophil and neutrophil early stages; and to classify myelogenic leukaemia with the active leucocytoses.= FOOTNOTES: [25] Naturally an ordinary leucocytosis may be combined with a lymphaemia. We have already mentioned elsewhere (see page 102) that in the leucocytosis of digestion or of diseases of the intestine in children, such a coincidence occurs. [26] The so-called agony leucocytosis we do not regard as a true leucocytosis, but only as the expression of a stoppage of the circulation caused by that condition. This produces an accumulation of the white corpuscles on the vessel walls, especially in the peripheral parts of the body which are as a rule used for clinical investigation. A leucocytosis is thus simulated.