[beta]. The prognostic importance of the enumeration of the white blood corpuscles. Thus for example the absence of leucocytosis influences the prognosis of pneumonia unfavourably (Kikodse and others); and the appearance of numerous myelocytes in diphtheria is ominous, as demonstrated by C. S. Engel (see page 78). * * * * * Finally, we may dismiss in a few words the origin of ~polynuclear neutrophil leucocytosis~, and refer to what has been said in another place on the function of the bone-marrow. In agreement with Kurloff's researches, Ehrlich formulated ("On severe anaemic conditions" 1892) his views on this subject as follows: "The bone-marrow is a breeding place in which polynuclear cells are produced in large numbers from mononuclear pre-existing forms. These polynuclear cells possess above all other elements the power of emigration. So soon as chemiotactic substances circulate in the blood, which attract the white elements, this power comes into play. This readily explains the rapid and sudden appearance of large numbers of leucocytes, which so many substances bring about, and particularly the bacterio-proteins, recognised by Buchner as leucocytic stimuli. I regard leucocytosis therefore, in agreement with Kurloff, as a function of the bone-marrow." Of great theoretical interest is the contrast between eosinophil and neutrophil cells. At the height of ordinary leucocytosis, the number of eosinophil cells is diminished often to disappearance; whilst during its decline they occur in abnormally high numbers. Hence it follows that the eosinophil and neutrophil cells must react towards stimulating substances completely differently, and in a certain sense oppositely[27]. It seems, generally speaking, that the bacterial =metabolic products formed in human diseases which are positively chemiotactic for the polynuclear neutrophil cells are negatively chemiotactic= for the eosinophils, and _vice versa_. The explanation of the individual clinical forms of leucocytosis is self-evident from the above description. The occurrence of physiological and inflammatory leucocytosis is exclusively to be explained by chemiotaxis. In the other forms, however, other factors also come into play, in particular the increased activity of the bone-marrow, or the extensive transformation of fatty to red marrow, causing a large fresh formation of leucocytes. [alpha] 2. ~Polynuclear eosinophil leucocytosis. Mast cells.~