The appearance of myelocytes in infectious diseases is particularly interesting. Rieder had previously demonstrated that myelocytes may be present in acute inflammatory leucocytoses; and recently a thorough work by C. S. Engel has appeared upon the occurrence of myelocytes in diphtheria. Engel discovered the interesting fact, that myelocytes are often to be found in children suffering from diphtheria, and further made the important observation that a high percentage of myelocytes (3.6-16.4% of the white elements) only occurs in severe cases, and points to an unfavourable prognosis. Myelocytes are also present in mild cases, though not constantly and in much smaller number. Türk has recently undertaken a very exact and thorough analysis of their occurrence in infectious diseases, in the course of which he accurately tabulated the white corpuscles in a large number of cases. The results he obtained in pneumonia are especially characteristic, for he found at the commencement of the disease that myelocytes are not seen at all or only very scantily: and it is only at the time of the crisis, or directly afterwards, that they become specially numerous. In isolated cases the increase at this time was very considerable; and in one case amounted almost to 12% of all neutrophil cells.
2. Mononuclear eosinophil cells ("=eosinophil myelocytes="). H. F. Müller was the first to point out their importance. They constitute the eosinophil analogue of the previous group, and are much larger than the polynuclear eosinophils; medium and small sized examples are often found in leukæmia. Eosinophil myelocytes are almost constantly present in myelogenous leukæmia and in anæmia pseudolymphatica infantum. Apart from these two diseases they are very rarely found; Mendel saw them for example in a case of myxoedema, Türk quite exceptionally in some infectious diseases.
3. =Small neutrophil pseudolymphocytes.= They are about as large as the small lymphocytes, possess a rounded deeply stained nucleus, and a small shell of protoplasm studded with a neutrophil granulation. The relatively deep stain of the nucleus and the small share of the protoplasm in the total cell body prevent confusion with the small forms of myelocytes, which never reach such small dimensions. The neutrophil pseudolymphocytes are exceedingly infrequent, and represent products of division of the polynuclear cells; they were first described by Ehrlich in a case of hemorrhagic small-pox. The process of division goes on in the blood in such a manner that the nuclear rod first divides into two to four single nuclei, and then the whole cell splits up into as many fragments. These cells occur also in fresh pleuritic exudations. After a time the nucleus of these cells becomes free, and the little masses of protoplasm thus cut off are taken up mostly by the spleen substance. The free nucleus likewise shares in the destruction. It is of the greatest importance that these cells, which up to the present have not elsewhere been described, should receive more attention. They must be of significance, in particular for the question of transitory hyperleucocytosis, which is by some referred to a destruction, by others to an altered localisation of the white blood corpuscles.