These two observations can only be interpreted by assuming a loss, in certain terminal stages, on the part of the organism, of its power of forming neutrophil substances. Similar conditions occur in non-leukaemic conditions; for example in a striking case of posthaemorrhagic anaemia described by Ehrlich. It is of great importance to direct attention to these cases, which up to the present have been practically disregarded--for ignorance of their occurrence may easily give rise to gross errors concerning the nature and origin of the mononuclear cells, and to the manufacture of a lienal form of leukaemia. * * * * * Finally we have to discuss the important question, how ~the origin of myelaemic blood~ is to be explained. According to our conceptions two possibilities come under consideration. Either we have to deal with a passive inflow of bone-marrow elements, or with an active emigration from the bone-marrow into the circulation. This important and difficult question is certainly not fully ripe for discussion. The most weighty objection to be raised against an active emigration of the bone-marrow cells is derived from the behaviour of the white blood corpuscles on the warm microscopic stage. These investigations have been performed by a number of authors of whom may be mentioned Biesiadecki, Neumann, Hayem, Loewit, Mayet, Gilbert, and particularly H. F. Mueller on the ground of his summary of this subject. Concerning the behaviour of the forms of cell here involved, all authors are agreed that under no conditions do the lymphocytes shew the smallest spontaneous movement; whilst the polynuclear neutrophil cells always exhibit vigorous contractility. With regard to the forms most characteristic of leukaemic blood the statements are partially contradictory. Some authors deny all spontaneous movement of these cells; but most of them report observations from which it follows that a certain power of spontaneous movement is not to be gainsaid. It will be admitted that in questions of this kind, negative results are weakened by positive data. Thus Jolly recently described similar observations as follows: "C'etaient des changements de forme sur place, lents et peu considerables, formations de bosselures a grands rayons, passage d'une forme arrondie a une forme ovulaire ou bilobee etc. Ces mouvements etaient visibles dans les observations I et IV et appartenaient surtout a des globules de grande taille." It is naturally impossible to decide if these minute movements suffice for a spontaneous locomotion. But one cannot exclude off-hand the supposition that they do. It is indeed supported by a further observation of Jolly on the mononuclear eosinophil cells of the marrow. Hitherto it was taken for established that these cells are completely devoid of spontaneous movement. Jolly however was recently able to examine a specimen from a case of typical leukaemia, =in which nearly all the eosinophil cells shewed active movement=.