SUSCEPTIBILITY AND IMMUNITY. 257 
The observations which first led Metschnikoff to adopt this view 
were made upon a species of daphnia which is subject to fatal infec- 
tion by a torula resembling the yeast fungus. Entering with the 
food, this fungus penetrates the walls of the intestine and invades the 
tissues. In certain cases the infection does not prove fatal, owing, as 
Metschnikoff asserts, to the fact that the fungus cells are seized upon 
by the leucocytes, which appear to accumulate around the invading 
parasite (chemiotaxis) for this special purpose. If they are success- 
ful in overpowering and destroying the parasite the animal recovers ; 
if not, it succumbs to the general infection which results. In a simi- 
lar manner, Metschnikoff supposes, pathogenic bacteria are destroyed 
when introduced into the body of an immune animal. The colorless 
blood corpuscles, which he designates phagocytes, accumulate at the 
point of invasion and pick up the living bacteria, as they are known 
to pick up inorganic particles injected into the circulation. So far 
there can be no doubt that Metschnikoff is right. The presence of 
bacteria in the leucocytes in considerable numbers, both at the point 
of inoculation and in the general circulation, has been repeatedly 
demonstrated in animals inoculated with various pathogenic bacteria. 
The writer observed this in his experiments, made in 1881, in which 
rabbits were inoculated with cultures of his Micrococcus Pasteuri ; 
and it was this observation which led him to suggest the theory 
which has since been so vigorously supported by Metschnikoff. But 
the presence of a certain number of bacteria within the leucocytes 
does not prove the destructive power of these cells for living patho- 
genic organisms. As urged by Weigert, Baumgarten, and others, 
it may be that the bacteria were already dead when they were picked 
up, having been destroyed by some agency outside of the blood cells, 
As heretofore stated, we have now experimental evidence that blood 
serum, quite independently of the cellular elements contained in it 
in the circulation, has decided germicidal power for certain patho- 
. genic bacteria, and that the blood serum of the rat and other animals 
which have a natural immunity against anthrax is especially fatal 
to the anthrax bacillus. 
Numerous experiments have been made with a view to deter- 
mining whether pathogenic bacteria are, in fact, destroyed within 
the leucocytes after being picked up, and different experimenters 
have arrived at different conclusions. In the case of mouse septi- 
czmia, already alluded to, and in gonorrhcea, one would be disposed 
to decide, from the appearance and arrangement of the pathogenic 
bacteria in the leucocytes, that they are not destroyed, but that, 
on the other hand, they multiply in the interior of these cells, which 
in the end succumb to this parasitic invasion. In both of the dis- 
eases mentioned we find leucocytes so completely filled with the 
17 
