estions
of modern medicine. An exhaustive account of the various works devoted
to it, of the methods and results, could fill by itself a whole volume,
and would widely exceed the limits of an account of the histology of the
blood. We can only deal fully therefore with the purely haematological
side of the subject.

Virchow designated by the name "=Leucocytosis=," a transient increase in
the number of the leucocytes in the blood; and taught that it occurred
in many physiological and pathological conditions. In the period that
followed particular attention was paid to the leucocytosis in infectious
diseases, and to the investigators of the last 15 years in this province
we owe very important conclusions as to the ~biological meaning~ of this
symptom. Above all Metschnikoff has done pioneer service in this
direction by his theory of phagocytes, and though his theory has been
shaken in many essential points, yet it has exercised a stimulating and
fruitful influence on the whole field of investigation.

To sketch Metschnikoff's doctrine in a few strokes is only possible by a
paraphrase of the very pregnant words "Phagocytes, digestive cells."
These words express the view, that the leucocytes defend the organism
against bacteria by imprisoning them by the aid of their pseudopodia,
taking them up into their substance, and so depriving them of the power
of external influence. The issue of an infectious disease would chiefly
depend on whether the number of leucocytes in the blood is sufficient
for this purpose.

This engaging theory of Metschnikoff has undergone important limitations
as the result of further investigation. Denys, Buchner, Martin Hahn,
Goldscheider and Jacob, Loewy and Richter, and many others have
demonstrated, that the most important weapon of the leucocytes is not
the mechanical one of their pseudopodia, but their chemical products
("Alexine," Bucher). By the aid of bactericidal or antitoxic substances
which they secrete, they neutralise the toxines produced by the
bacteria, and thus render the foe harmless by destroying his weapon of
offence, even if they do not exterminate him.

An explanation of the almost constant increase of the leucocytes of the
blood in bacterial diseases is given by the chemiotactic as well as by
the phagocytic theory of leucocytosis. The principle of =chemiotaxis=
discovered by Pfeffer asserts that bacteria, or rather their metabolic
products, are able to attract by chemical stimu