Very important conclusions on the interesting question of leucocytosis can be drawn from these observations. Bearing in mind that polynuclear neutrophil cells are developed and stored up only in the bone-marrow, that in ordinary leucocytosis only the polynuclear forms are increased in the blood-stream, it is evident that leucocytosis is purely a function of the bone-marrow, as Ehrlich has always insisted with all distinctness. It is only on this assumption that the frequently sudden appearance of leucocytosis, as has so often been observed in morbid and experimental conditions, can be satisfactorily explained. In these cases the space of time, amounting often only to minutes, is far too short for a new formation of leucocytes to be conceivable; there must be places in which these cells are already completely formed, and able thence to emigrate on any suitable stimulus. This place is single, and is the bone-marrow alone. Here all mononuclear forms gradually ripen to the polynuclear contractile cells, which obey each chemiotactic stimulus by emigration, and which thus bring about sudden leucocytosis.
The bone-marrow thus fulfils, amongst others, the extremely important function of a protective organ, by which definite injurious influences which affect the organism may be quickly and energetically combated. Just as in a fire-station ample means of assistance is continuously in readiness immediately to answer an alarm from any quarter.
We wish to insist once more, that the large mononuclear leucocytes and the transitional forms of the normal blood are not concerned in the increase in ordinary leucocytosis; in leucocytosis of high degree their relative number may indeed be lowered, in consequence of the exclusive increase of the polynuclear cells. It appears then that these elements do not react to chemiotactic stimuli, and that possibly they reach the blood by entirely different ways than the polynuclears do.
We believe that these non-granulated mononuclear cells of man are to be regarded as analogous to those of the guinea-pig described by Kurloff (see page 86). The mononuclear cells of man however are finally transformed into the neutrophil granulated cells, whilst the cells of Kurloff remain free from granules in the course of their metamorphosis. In acute leucocytosis in the guinea-pig only the pseudo-eosinophil polynuclear cells are increased, which wander as such out of the bone-marrow, but not the polynucleated non-granulated forms, which but slowly grow to maturity in the blood. Thus the peculiarities of guinea-pig's blood, in which two kinds of polynuclear cells are recognisable, throw light upon the corresponding conditions in human blood. The distinction in the latter is more difficult, since it is not evident in this case that the fully formed polynuclear neutrophil leucocytes have a twofold origin: for the majority wander fully formed from the bone-marrow into the blood, and only a considerably smaller number grow to maturity within the blood-stream from the mononuclear and transitional forms.
No definite statement can as yet be made as to the places of formation of the non-granulated large mononuclear leucocytes.
Kurloff has demonstrated, that in the guinea-pig these cells are present both in the bone-marrow and in the spleen, but that after extirpation of the spleen the absolute number does not change. The bone-marrow then in the guinea-pig can also preserve the balance of the large mononuclear, non-granular cells in the blood.
The numbers we found in our blood investigations in man after splenectomy were also normal. We may then doubtless assume that the large mononuclear granuleless cells of human blood also arise for the most part from the bone-marrow. In this tissue they are to be picked out in the medley of the different kinds of cells only with the utmost difficulty, owing to their small number and their but little characteristic properties. Consequently an exact investigation of their origin could probably only be successful if it were possible experimentally to produce a disease in which these forms in particular underwent important increase. This advance is not quite hopeless, since in man at least an absolute increase of the large mononuclear cells is observed in the post-febrile stage of measles.
On the grounds merely of microscopical investigations we conclude that the bone-marrow is by far the most important of the blood-forming organs, for its function is the exclusive production of red blood discs as well as of the chief group of the white corpuscles, the polynuclear neutrophil.
The physiological, experimental investigation of the functions of the bone-marrow offers insurmountable difficulties. An exclusion of the whole bone-marrow or of larger portions only is an impossible operation. Nor can we ascribe any value to the researches which endeavour to obtain a result by comparative enumerations of the arterial and venous blood of a bone-marrow area. J. P. Roietzky working under Uskoff's direction has recently made counts of this kind in the dog, from the nutrient artery of the tibia and the corresponding vein. He found that the number of white corpuscles of the vein is slightly greater, that on the other hand the absolute number of "young corpuscles" (Uskoff), i.e. of the lymphocytes, has been considerably diminished, whilst the number of "ripe" corpuscles, which for the most part correspond to our polynuclear, is considerably increased. He gives the following table:
| Total number | Young corpuscles | Ripe corpuscles | Old corpuscles |
| Arterial blood 15000 | 1950 (13%) | 840 (5.6%) | 12210 (81%) |
| Venous " 16400 | 656 (4.0%) | 2788 (17.0%) | 12956 (79.0%) |