972 
this we very soon see that the equilibrium which was disturbed is 
restored again. 
A similar local mingling of blood and tissue might be accepted 
for bone-marrow under the influence of a permanently existing 
stimulus which retains and rejuvenates the elements of the blood. 
Myeloblasts therefore in accordance with this conception could only 
be formed from so-called undifferentiated cells there where definite 
proporties of the blood are still present, therefore in the immediate 
vicinity of, perhaps also in the blood vessels; to all appearances 
the factors for this are more markedly present if many lymphoid 
cells accumulate locally combined with an abundant blood supply, 
and it does not happen at all there, where any important influence 
of the blood fails, as in lymph glands. In this way it can be ex- 
plained why the metaplasia of the spleen into myeloid tissue is 
never observed in the Malpighian corpuscles but always in the pulpa 
which is rich in blood. In the pulpa the elements of blood and 
connective tissue are indiscriminately mished. On the ground of what 
has been mentioned above, it seems us better, in the case of the 
different cell types of mesenchymal origin, not to speak of strongly 
separated specific cell-types, for which there is no possibility of 
transition in one another; we should prefer to say, that all the 
different cell forms here are the result of the action of the varying 
conditions of the surrounding fluid, which on its turn is continually 
changed through the living cells; in the state of equilibrium issuing 
from this, perhaps one cell-type will prevail, but a modification of 
the fluid will at the same time cause a change in this equilibrium, 
so as to make that perhaps another form of cell will get the upper 
hand; only for the granulated cells, if once formed, there is no 
possibility for a reversal process. According to the presence of two 
chief liquids in the body, differing widely, it is reasonable, that we 
also met with two chief types of cells of which one prevails in one, 
the other in another medium; thus the bone-marrow shows normally 
a transformation into granulated cells, while the spleen consists 
principally of lymphoid tissue, although by no means exclusively. 
Under changes, due to pathological changes in the milieu, the 
development of the cells can be modified, as happens in the case 
of the different forms of leukaemia. 
SUMMARY. 
What has been discussed above can be summarised as follows: 
The total number of white blood corpuscles that 
