5. =The eosinophil cells.= These are characterised by a coarse, round granulation, staining deeply with acid dyes, and similar in other respects to the polynuclear neutrophils. With faint staining, a thin peripheral layer of the eosinophil granule is seen more deeply stained than the interior. The nucleus as a rule is not so deeply stained as in the polynuclear neutrophil, but otherwise in its general shape is completely similar. Both forms have in common a considerable contractility, which renders possible their emigration from the vessels, and their appearance in exudations and in pus. The size of the eosinophils frequently exceeds that of the neutrophils. Their number is normally about 2-4% of the white cells. 6. The mast cells. These are present, though very sparingly, in every normal blood; 0.5% is their maximum number in health. Their intensely basophil granulation, of very irregular size and unequal distribution, must specially be mentioned. The granulation possesses the further peculiarity, in that with the majority of basic dyes it stains, not in the pure colour of the dye, but metachromatically--most deeply with thionin. As Dr Morgenroth found, the deviation from the colour of the dye is still more marked with Kresyl-violet-R (Muelheim manufactory), when the granules stain almost a pure brown. The staining power of the nuclei is very small, and it is therefore hard to make out the shape of the nucleus without the use of difficult methods. In triacid preparations the granulation is unstained, and the mast cells appear as clear, polynuclear cells, free from granules. * * * * * So much for the colourless cells in the blood of the normal adult. In pathological cases, not only do the forms so far mentioned occur in altered numbers, but abnormal cells also make their appearance. To these belong: 1. =Mononuclear cells with neutrophil granulation.= ("=Myelocytes=," Ehrlich.) Generally they are bulky, with a relatively large, faintly staining nucleus, often fairly centrally placed, and equally surrounded by protoplasm on all sides. A fundamental distinction from the large mononuclear cells lies in the fact that the protoplasm exhibits a more or less numerous neutrophil granulation. Besides the larger myelocytes, much smaller forms, approximating to the size of the erythrocytes are also found. All transitions between these two stages are likewise met with. In contradistinction to the polynuclear neutrophil elements, these mononuclear forms shew no amoeboid movement on the warm stage. They form a constant characteristic of myelogenic leukaemia, and in these cases generally occur in large numbers.