189 
was scrubbed several times with ether and alcohol and a small incision 
made with a sterile knife through the dermis. The cultures were then 
made from the drops of blood which oozed from the incised wound. 
Usually, when the injection is made beneath the skin of the abdomen, 
after a few hours an cedematous swelling, which may not entirely disap- 
pear for from twelve to twenty-four hours, appears near the point of 
inoculation. 
The different series of cultures made from the animals have shown 
that six to eight hours from the time of inoculation the organisms are 
still very numerous in the tissues, after which time they gradually 
disappear, so that the cultures made twenty-four hours subsequent to the 
injections remain sterile. It seems probable that the more resistent 
organisms are those which remain alive the longest, and that there is 
here a true survival of the fittest. A trial is therefore being made to 
ascertain whether it is possible to increase the virulence of these atten- 
uated pest strains by such a procedure. As soon as the cultures made 
from the blood of one animal develop, they are inoculated into another 
monkey, and so on through a long series. It seems possible that such a 
method may have certain advantages over that in which the organisms 
are inclosed in celloidin sacs and placed in the abdominal cavity of 
animals. Kolle and Otto have shown that it is very difficult or impos-. 
sible to increase the pathogenesis of more virulent strains of pest bacilli '* 
by repeated passages through guinea pigs; but this need not necessarily 
be true with more avirulent strains.** 
It may be questioned whether the organism which has been employed 
in these human inoculations is really a strain of Bacillus pestis. There- 
fore, while it is not considered necessary in detail here to relate the 
immunity reactions, morphology, etc., of this organism, it may be stated 
that unquestioned proof of this hes in the fact that I have vaccinated 
both guinea pigs and large numbers of monkeys with this culture and have 
later shown them to possess high and undoubted pest immunity by 
subsequently inoculating them with large amounts of virulent plague 
bacilli. Indeed, with no other method of inoculation have I been able 
to obtain such favorable results as with this pest vaccine. 
Allusion has been made in other places to the similarity between 
certain of the immunity reactions of plague and of rinderpest. The 
present method of human vaccination against plague may be compared 
with that first recommended by Robert Koch in the immunization of 
cattle against rinderpest. Koch used the bile of animals which had died 
of this disease and in which perhaps the living attenuated organism of 
rinderpest exists. Doubtless a higher immunity in man against plague 
could be obtained were a more virulent culture of the pest bacillus 
1®'Those which kill the animal. 
“ Those which are not capable of causing the death of the animal, even in large 
amounts, 
