Few facts in medicine are as well established as the increase in leukocytes and neutrophils that characterizes certain infectious processes, although the information to be gained from a study of the morphologic characteristics of the leukocytes responsible for this increase has not received due consideration. In the development of an adult polymorphonuclear neutrophil, the young cell passes through a series of well defined stages, with gradually increasing condensation and segmentation of the nuclear chromatin and the appearance of granules and neutrophilic staining properties in the cytoplasm, until the mature neutrophil is formed. The observation that there is an increase in the number of the less mature polymorphonuclear neutrophils in certain acute infections is the basis for the various methods of classification of these cells which have appeared from time to time during the past thirty years.Arneth 1 is credited with the pioneer work on the changes in nuclear structure of the neutrophil under various conditions. In 1904 he published his classification in which he divided these cells into five main groups, each corresponding to the number of segments in the nucleus. He noted the increase in the nonsegmented or less mature forms during acute infections. The complexity of his classification prevented its widespread adoption, and it was not until 1920 that Schilling -modi¬ fied Arneth's system to a more practical form. He grouped each kind of leukocyte separately and then subdivided the neutrophils into four groups, three of which comprised cells with nonsegmented nuclei and the fourth, those in which the nuclear segments were joined by a thin, hairlike strand of chromatin known as a filament. In the order of their maturity the three types of nonsegmented forms were classified as myelocytes, with round nuclei, metamyelocytes, with indented nuclei, and band forms, with bilobed nuclei the segments of which were joined by a rather broad band of chromatin. Schilling called the increase in
