4. The (so-called) "=polynuclear leucocytes=." These arise in small part, as will be described later in detail, from the above-mentioned No. 3, within the blood stream. By far the larger part is produced fully formed in the bone-marrow, and emigrate to the blood. These cells are rather smaller than Nos. 3 and 2 and are distinguished by the following peculiarities: firstly by a peculiar polymorphous form of nucleus which gives the relatively long, irregularly bulged and indented nuclear rod the appearance of an S, Y, E or Z. The complete decomposition of this nuclear rod into three to four small round single nuclei may occur during life, as a natural process. Ehrlich first discovered it in a case of haemorrhagic small-pox; it is frequently found in fresh exudations. Formerly when various reagents, for instance acetic acid, were customarily used, the decomposition of the nucleus into several parts was more frequently observed, and Ehrlich for this reason chose the not wholly appropriate name "polynuclear" for this form of cell. As this name has now been universally adopted, and misunderstandings cannot be expected, it is undoubtedly better to keep to it. The expression "Cells with polymorphous nuclei" would be more accurate. [Illustration: Fig. 3. Nucleoli in larger lymphocytes. (From a photograph of a preparation from chronic lymphatic leukaemia.) _To face page 74_] The nucleus stains very deeply with all dyes; the protoplasm possesses a strong attraction for most acid stains, and is unmistakeably characterised by the presence of a dense neutrophil granulation. The reaction of the protoplasm is alkaline, to a less degree however than in the lymphocytes. No free glycogen is contained in the polynuclear cells as a rule; nevertheless in certain diseases cells are always found which give a marked iodine reaction. In this manner the appearance of cells containing glycogen in diabetes was first proved. (Ehrlich, Gabritschewsky, Livierato.) The iodine reaction in the white blood corpuscles is also seen in severe contusions and fractures, in pneumonias, in rapidly progressing phlegmata from streptococcus and staphylococcus, after protracted narcosis (Goldberger and Weiss). Ehrlich explains the appearance of glycogen as follows. The glycogen is not present in the cell as such, but in the form of a compound, which does not stain with iodine. This compound readily splits off glycogen, which then gives the iodine reaction[12]. We cannot regard the perinuclear green granules, described by Neusser in the polynuclear cells, as pre-existing. (See p. 42.) The number of polynuclear leucocytes in the blood of the healthy adult amounts to about 70-72%, of the total white corpuscles. (Einhorn.)[13]