SUSCEPTIBILITY AND IMMUNITY. 245 
Again, cultivation within the body of a living animal may, in 
certain cases, cause a diminution in the virulence of a pathogenic 
microérganism. Thus Pasteur and Thuiller have shown that the 
microbe of rouget when inoculated into a rabbit kills the animal, but 
that its pathogenic virulence is nevertheless so modified that a cul- 
ture made from the blood of a rabbit killed by it is a suitable “‘ vac- 
cine” for the pig. 
On the other hand, we have experimental evidence that the viru- 
lence of attenuated cultures may be reéstablished by passing them 
through the bodies of susceptible animals. Thus a culture of the 
bacillus of rouget, attenuated by having been passed through the 
body of a rabbit, is restored to its original virulence by passing it 
through the bodies of pigeons. Anda culture of the anthrax bacillus 
which will not kill an adult guinea-pig may be fatal to a very young 
animal of the same species or to a mouse, and the bacillus cultivated 
from the blood of such an animal will be found to have greatly in- 
creased virulence. 
In Pasteur’s inoculations against anthrax “ attenuated ” cultures 
are employed which contain the living pathogenic germ as well as 
the toxic products developed during its growth. Usually two inocu- 
lations are made with cultures of different degrees of attenuation— 
that is to say, with cultures in which the toxic products are formed 
in less amount than in virus of full power. The most attenuated 
virus is first injected, and after some time the second vaccine, which 
if injected first might have caused a considerable mortality. The 
animal is thus protected from the pathogenic action of the most 
virulent cultures. 
Now, it has been shown by recent experiments that a similar im- 
munity may result from the injection into a susceptible animal of the 
toxic products contained in a virulent culture, independently of the 
living bacteria to which they owe their origin. Chauveau, in 1880, 
ascertained that if pregnant ewes are protected against anthrax by 
inoculation with an attenuated virus, their lambs, when born, also 
give evidence of having acquired an immunity from the disease. As 
the investigations of Davaine seemed to show that the anthrax 
bacillus cannot pass through the placenta from the mother to the 
foetus, the inference seemed justified that the acquired immunity of 
the latter was due to some soluble substance which could pass the 
placental barrier. More recent researches by Strauss and Chamber- 
lain, Malvoz and Jacquet, and others, show that the placenta is not 
such an impassable barrier for bacteria as was generally believed at 
the time of Chauveau’s experiments, so that these cannot be accepted 
as establishing the inference referred to. But, as stated, we have 
more recent experimental evidence which shows that immunity may 
