, a leukaemia of myelogenic nature is nearly always
present. (For the very rare exceptions to this rule, which it may be
added can never be confused with leukaemia, see pages 77, 78.)

Exactly similar conditions hold good for the eosinophil cells, in as
much as the singly nucleated forms, which one may call eosinophil
myelocytes, occur, almost exclusively, in leukaemic blood. These forms,
which were first recognised by H. F. Mueller, are however of less
importance, for in myelogenic leukaemia the chief part of the foreign
admixture of the blood is made up of Ehrlich's myelocytes.

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 th