The classification of the white corpuscles of normal human blood, drawn up by Ehrlich, has been accepted by most authors, and we therefore give a short summary of it, as founded on the dry specimen. 1. =The Lymphocytes.= These are small cells, as a rule approximating in size to the red blood corpuscles. Their body is occupied by a large round homogeneously stained nucleus centrally situated, whilst the protoplasm surrounds the nucleus as a concentric border. Between nucleus and protoplasm there is often found a narrow areola, which doubtless results from artificial retraction. Nucleus and protoplasm are basophil, nevertheless in many methods of staining the protoplasm possesses a much stronger affinity for the basic stain than does the nucleus. The nucleus in these cases stands out as a bright spot from the deeply stained mass of protoplasm, which is reticulated in a peculiar manner. Within the nucleus are often to be found one or two nucleoli with a relatively thick and deeply stained membrane. With methylene blue and similar dyes the protoplasm stains unequally, which is not to be considered as the expression of a granulation, as Ehrlich first assumed, but rather of a reticular structure. The contour of the lymphocytes is not quite smooth as a rule, at least in the larger forms, but is somewhat frayed, jagged, and uneven (Fig. 1). Small portions of the peripheral substance may repeatedly bud off, especially in the large forms, and circulate in the blood as free elements. In stained specimens, especially from lymphatic leukaemia, these forms, which completely resemble the protoplasm of the lymphocytes in their staining, may from their nature and origin be readily recognised. As far as the further metamorphosis of the nucleus is concerned, a sharp notching of the border of the nucleus may occasionally be found, the further fate of which is shewn in the accompanying figure (Fig. 3). It is evident that in this case the resulting nuclear forms are quite different from those which are characteristic of the polynuclear elements. The protoplasm possesses no special affinity for acid and neutral dyes, and hence in triacid and haematoxylin preparations the small lymphocytes are seen chiefly as lightly stained nuclei, apparently free. In the larger cells the protoplasm can be seen even in these preparations to be slightly stained. By the aid of the iodine-eosine method the reaction of the protoplasm of the lymphocytes is shewn to be strongly alkaline. They do not contain glycogen.