ter of the subject. That
the white corpuscles play a significant part in the physiology and
pathology of man has been recognised but slowly, obviously because there
was at first some hesitancy in ascribing important functions to elements
that are present in the blood in such relatively small numbers. A place
in pathology was first assured to them by Virchow's discovery of
leukaemia. The interest in the question was increased by Cohnheim's
discovery that inflammation and suppuration are due to an emigration of
the white blood corpuscles, and these conditions were particularly
suitable for throwing light on normal processes. The fact that in
diffuse inflammations, large quantities of pus are often produced in a
short time, without the blood being thereby made poorer in
leucocytes,--that the opposite indeed occurs,--necessitated the
supposition that the source of the leucocytes must be extraordinarily
productive. Hence in contradistinction to the red blood corpuscles,
their small number is fully compensated by their exceptional power of
regeneration.

Nevertheless, a considerable time elapsed before the powerful impulse
that started from Cohnheim, bore fruit for clinical histology. As we
have mentioned this was due to the circumstance that an exact
differentiation of the various forms of leucocytes was very difficult
with the methods in use up to that time. Although such distinguished
observers as Wharton Jones and Max Schultze had been able to distinguish
different types of leucocytes, Cohnheim's work remained clinically
fruitless since the criteria they assigned were far too subtle for
investigation at the bedside. Virchow indeed, the discoverer of
leucocytosis, interpreted it as an increase of the lymphocytes; whereas
it is chiefly produced by the polynuclear cells. Only after the
distinction was facilitated by the dry preparation and the use of
stains, did interest in the white corpuscles increase, and continue
progressively to the present day. This is borne out by the exceptionally
exhaustive haematological literature, and particularly by that of
leucocytosis.

In spite of these advances, a retrograde movement in the doctrine of the
leucocytes has gained ground surprisingly, especially in the last few
years. Ever since Virchow's description of the lymphocytes, observers
have tried to separate the various forms of leucocytes one from another,
and if possible to assign different places of origin to these different
ki