In favour of the diagnostic value of the absolute increase of the eosinophil cells are those cases too, where with a blood condition closely recalling leukaemia, the absence of eosinophil cells excludes the diagnosis of that disease. In a case of carcinoma of the bone-marrow, described by Epstein, with an anaemic constitution of the blood (nearly always present it may be mentioned in leukaemia), there was found a marked increase of the white blood corpuscles, numerous neutrophil myelocytes and nucleated red corpuscles. Anyone holding, as Mueller and Rieder do, that the number of eosinophil cells need not be considered in the diagnosis, must in this case have diagnosed myelogenic leukaemia. This however was according to Ehrlich's system impossible owing to the complete absence of eosinophil cells. From all these observations it follows that an absolute increase of eosinophil cells is indispensable for the diagnosis of leukaemia. 4. =The absolute increase of the mast cells.= The mast cells are always increased in myelogenic leukaemia. They may be counted in leukaemic blood with the aid of the triacid or eosine-methylene blue stain. As shewn by the former they appear as polynuclear cells free from granules, since their granulation takes on no dye of the triacid mixture. In all cases of myelogenic leukaemia the increase of mast cells is absolute and considerable. Generally they are equally or half as numerous as the eosinophils, occasionally they may exceed the latter in number. Hence it follows that the mast cells undergo an increase in number relatively greater than the eosinophil cells, for they normally amount only to some 0.28%. They are perhaps of greater diagnostic value than the eosinophils, because up to the present time we know of no other condition (in contradistinction to eosinophil leucocytosis) in which a marked increase of the mast cells occurs. 5. =Atypical forms of the white corpuscles.= Amongst these are to be mentioned: (_a_) dwarf forms of the polynuclear neutrophils and of the eosinophil elements respectively. As a rule they resemble normal polynuclear cells on a small scale. (_b_) Dwarf forms of the mononuclear neutrophil and eosinophil leucocytes, which correspond to the pseudo-lymphocytes described elsewhere (see p. 78). The importance of these dwarf forms for leukaemia is as yet insufficiently explained; and it is difficult to decide whether they are already small forms on reaching the blood-stream, or whether they are there produced by division of a large cell. (_c_) =Cells with mitoses=. Formerly particular weight was laid on the observation of mitoses, for they were regarded as evidence that the increase of white blood corpuscles was brought about in the circulating blood itself, an assumption specially supported by Loewit.