258 SUSCEPTIBILITY AND IMMUNITY. 
pathogenic microérganisms that it is difficult to believe that they 
have all been picked up by a voracious phagocyte, which has 
stuffed itself to repletion, while numerous other leucocytes from 
the same source and in the same microscopic field of view have 
failed to capture a single bacillus or micrococcus. Moreover, the 
staining of the parasitic invaders, and the characteristic arrange- 
ment of the “gonococcus” in stained preparations of gonorrhceal 
pus, indicate that their vitality has not been destroyed in the interior 
of the leucocytes or pus cells, and we can scarcely doubt that the 
large number found in certain cells is due to multiplication in situ 
rather than to an unusual activity of these particular cells. But in 
certain infectious diseases, and especially in anthrax, the bacilli in- 
cluded within the leucocytes often give evidence of degenerative 
changes, which would support the view that they are destroyed by 
the leucocytes, unless these changes occurred before they were picked 
up, as is maintained by Nuttall and others. We cannot consider 
this question as definitely settled. 
Going back to the demonstrated fact that susceptible animals may 
be made immune by inoculating them with the toxic products pro- 
duced during the growth of certain pathogenic bacteria, we may 
suppose either that immunity results from the continued presence of 
these toxic products in the body of the inoculated animal, or from a 
tolerance acquired at the time of the inoculation and subsequently 
retained—by transmission from cell to cell, as heretofore suggested. 
Under the first hypothesis—retention theory—immunity may be ex- 
plained as due to a continued tolerance on the part of the cellular ele- 
ments of the body to the toxic substances introduced and retained ; 
or to the effect of these retained toxic products in destroying the 
pathogenic bacteria, or in neutralizing their products when these are 
subsequently introduced into the body of the immune animal. We 
eannot understand how toxic substances introduced in the first in- 
stance can neutralize substances of the same kind introduced at a 
later date. There is something in the blood of the rat which, accord- 
ing to Behring, neutralizes the toxic substances present in a filtered 
culture of the tetanus bacillus ; but whatever this substance may be, 
it is evidently different from the toxic substance which it destroys, 
and there is nothing in chemistry to justify the supposition last 
made. Is it, then, by destroying the pathogenic microérganism 
that these inoculated and retained toxic products preserve the animal 
from future infection ? Opposed to this supposition is the fact that 
the blood of an animal made immune in this way, when removed 
from the body, does not prove to haveincreased germicidal power as 
compared with that of a susceptible animal of the same species. 
Again, these same toxic substances in cultures of the anthrax bacillus, 
