These properties taken as a whole constitute a picture completely characteristic of the lymphocytes; and these elements can thereby be diagnosed and separated from other forms, even when their size varies. Generally speaking, these cells, as above mentioned, are distinguished in the blood of the healthy adult by their small size, approximating to that of the red blood corpuscles. In the blood of children on the contrary larger forms are found even in health; and in lymphatic leukaemia particularly large forms occur, which are mistaken in various ways by unpractised observers. Thus Troje's "marrow cells" still figure in the literature, but have absolutely nothing to do with the marrow. They are large lymphocytes, as was established by A. Fraenkel years afterwards. [Illustration: Fig. 1. Fraying out of the protoplasmic border in large lymphocytes. Free plasma elements formed by budding. ("Plasmolysis.") (From a photograph of a preparation from chronic lymphatic leukaemia.) _To face page 72_] [Illustration: Fig. 2. (From Rieder's Atlas.) Metamorphosis of the nucleus of the lymphocytes. (Combined picture from a preparation from acute leukaemia.) _To follow Fig. 1_] In the normal blood of adults the number of the lymphocytes amounts to about 22-25% of the colourless elements. Increase of the lymphocytes alone occurs, but in comparison with that of the other forms, much more seldom, and will be conveniently called by the special names of "lymphocytosis" or "lymphaemia." 2. Sharply to be distinguished from the lymphocytes is the second group: the "large mononuclear leucocytes." These are large cells about twice to three times the size of the erythrocytes. They possess a large oval nucleus, as a rule eccentrically situated and staining feebly, and a relatively abundant protoplasm. The latter is free from granulations, feebly basophil, and in contrast to the protoplasm of the lymphocytes stains less deeply than the nucleus. This group is present in normal blood in but small numbers (about 1%). They are separated from the lymphocytes because they are totally different in appearance, and because forms transitional between the two are not observed. It cannot yet be decided from which blood-producing organs these forms arise, whether from spleen or bone-marrow, although there are many reasons for regarding the latter as their place of origin. These large mononuclear leucocytes change in the blood to the following kind: 3. "=The transitional forms.=" These resemble the preceding, but are distinguished therefrom by deep notchings of the nucleus, which often give it an hour-glass shape, further by a somewhat greater affinity of the nucleus for stains, and by the presence of scanty neutrophil granulations in the protoplasm. The groups 2 and 3 comprise together about 2-4% of the white blood corpuscles[11].