674 
C. FISCH. 
to- be abandoned as decidedly dangerous. I need not enlarge on | 
them in this paper, because they are well-known things to my ; 
readers. 
The reasons why these attempts as well as some others to 
establish passive immunity by injection of serum of immunized 
animals, have failed are to be looked for in several directions. 
In the first place quite a number of investigators have conducted | 
their experiments with germs that were not hog cholera germs | 
at all; I already mentioned Metschnikoff as laboring under j 
such a quid pro qtto^ and it seems that Selander too has fallen a ! 
victim to the very same error. Certainly a great many failures 
in this country are due to it. In the second place, very often 
the biologic qualities of the material (cultures) used were such ! 
as to make the possibility of a success questionable from the | 
beginning; one peculiarity of the ho^g cholera bacillus is to i 
rapidly lose its pathologic character and virulence, and to be- | 
come an almost harmless parasite. On the other side, an i 
increase of this virulence is very difficult to obtain. That j 
a great number of investigators have experimented with a i 
bacillus of very low virulence is shown by the large doses i 
used, by the protracted course of the disease in the animals i 
experimented upon, and finally by the relative insusceptibility 
of some animals (chickens) for these bacilli. i 
I began my experiments with a hog cholera culture which I ! 
owe to the kindness of Dr. Welch. When I attempted to de- j 
termine its virulence I found that, a suspension of 3 mg. of a 48 ! 
hours agar culture in physiologic salt solution, subcutaneously ; 
injected was necessary in order to kill a 150O" to 2000-gramme : 
rabbit in from 6 to 8 days. To kill guinea-pigs I had to 
use much larger doses. My first efforts, therefore, were directed i 
towards an increase of this virulence. The usual methods did ; 
not achieve this aim to any appreciable extent. After long : 
experimenting I succeeded, at last, by injecting a large dose of ' 
fresh culture (suspension made with normal salt solution) into . 
the abdominal cavity; after death one c. c. of the peritoneal 
exudate was administered intraperitoneally to another rabbit, , 
i 
