clumped together in masses, the bodies became swollen, broken up, and
finally disintegrated. This property of the serum was described as due
to a substance in the serum called _alexine_, which in the immune
animal became greatly increased in amount. It was even denied by some
that phagocytosis of living bacteria took place, and that all those
included in the cells were dead, having been destroyed in the first
instance by the serum. The strife became a national one between the
French and Germans,--on the one side in France the phagocytic theory
was defended, and in Germany, on the other, the theory of serum
immunity. The mass of experimental work which poured from the
laboratories of the two countries in attack and defence became so
great that it could not easily be followed. It had a good influence
because, without the stimulation of this national rivalry, the
knowledge which gradually arose from this work would not have been so
quickly acquired. It is interesting that the mode of action of the
serum in destroying bacteria was demonstrated not by a German but by
Bordet, a French observer and a pupil of Metschnikoff. He showed that
the serum contained two distinct substances, each necessary for the
destructive action. The separate action of these substances can be
studied since one is _thermolabile_, or destroyed by heating the
serum to one hundred and thirty-three degrees; the other
_thermostabile_, or capable of withstanding a greater degree of
heat. These substances are known only by their effect, they have never
been separated from the serum. The thermostabile substance, or
_amboceptor_, as it is generally called, has in itself no
destructive action on the bacteria; but in some way so alters them
that they can be acted on by the thermolabile substance called
_complement_ whose action is destructive. The amount of
amboceptor may increase in the course of infection and its formation
stimulated, the amount of complement remains unchanged. The action of