820 The American Naturalist. 
from when the phagocytes overmaster the exciting causes, and immu- 
nity after one attack or after inoculation depends on the phagocytes 
having become accustomed to combat the micro-organism. 
This theory is supported by Metschnikoff 's observations on Daphniae 
which are attacked by a Torula with needle-like ascospores. These 
latter having been swallowed penetrate the tissues ; as soon as this 
happens a leucocyte appears, and the spores are enveloped and 
destroyed. If the spore remain unattached and germinate the animal 
is infected. In frogs, too, anthrax bacilli are taken up by leucocytes 
and destroyed. At a temperature of about 30° Cent, only a few 
leucocytes take up the bacilli, and the animals become infected. This 
is explained on the hypothesis that the anthrax bacilli are more potent 
at this temperature, owing to their being accustomed to dealing with 
warm-blooded leucocytes [in the sheep]. In warm-blooded animals 
Metschnikoff rarely found bacilli in the leucocytes, but if the animals 
had been protected by a weakened virus the bacilli were picked up in 
quantities and destroyed. Hence it is concluded that immunity is 
derived from the leucocytes having got used to the poison of the 
bacteria. 
Bacteria-eating phagocytes were also found in erysipelas and relaps- 
ing fever, and are also assumed to be present in gonorrhoea, leprosy, 
and tuberculosis. 
According to Hess, the phagocytic privilege is shared by the cells of 
the splenic parenchyma, and of the liver, and Ribbert asserts that the 
spores of various kinds of Aspergillus and Mucor are got rid of in a 
similar manner. If, however, many spores be injected, the number of 
leucocytes may not suffice to prevent their development, and this last- 
mentioned author also believes that the viability of the fungi is 
diminished by the leucocytes cutting off the supply of oxygen. Other 
facts in support of the theory are, that if an animal survive the intro- 
duction of a small quantity of spores, there will be found, on a second 
injection, a much larger number of leucocytes, and that, as stated 
by Lubarsch, anthrax bacilli killed by boiling are not so quickly 
taken up by leucocytes in the frog as when injected in the living 
Against the theory are ranged numerous writers and experimenters, 
among whom may be mentioned Baumgarten and Weigert, who, while 
accepting the data, doubt the interpretation of the facts and the 
correctness of the hypothesis. Experiments made by C. D. Holmfield 
showed that only a few bacteria were taken up by leucocytes, and that 
the greater number of bacteria were destroyed outside the cells. 
