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184 
pest culture, when all proved to be immune. Only two control animals 
were inoculated in each of these series. This number was hardly suf- 
ficient to render the experiments conclusive. ‘The authors do not state 
the size of the dose, the virulence of the organism used in testing the im- 
munity, and the method of inoculation. When this same attenuated 
bacillus was allowed to grow continuously on artificial media during 
forty to fifty days it was claimed that its virulence was greatly decreased, 
since of thirty’rats inoculated with such a culture, none died. Later, 
on testing fourteen of these animals with the virulent pest organism, 
five succumbed. Seven apes were inoculated with another plague culture 
which, when injected into rats, killed from 40 to 50 per cent of these 
animals. None of the apes died from the effects of the vaccination. 
Apparently only two of these animals were subsequently tested for their 
immunity; these developed localized buboes, but recovered. Since the 
single ape used for control purposes also did not die we can draw 
almost no conclusions from these experiments. The amount of the 
organism employed in the vaccination or in the testing of the immunity 
is not stated. Yersin apparently was inoculated with the most attenuated 
culture (fifteen days old), but the size of the dose used in the vaccination 
is not given. Only very slight symptoms developed. At the same time, 
ten rats were injected with this culture but none died. The immunity 
of these animals had not been tested at the time the paper of these 
authors appeared. Although these experiments were reported in 1900, 
I have been unable to find any further reference made to them since 
that date either by Yersin or his colleagues. 
No extensive or convincing experiments in regard to the value of the 
employment of the living attenuated cultures in the immunization even 
of animals against plague had apparently been undertaken until Kolle 
investigated this subject. 
In 1902 and 1903 Kolle and Otto '* inoculated eighteen guinea pigs 
subcutaneously with an attenuated culture of the pest bacillus. The 
organism was an old laboratory culture in which the reduction of the 
virulence had, in some unknown manner, taken place during its growth 
on artificial media. Buboes, which later discharged and healed and in 
the pus from which a few bacilli were present, developed in the animals, 
but they showed no other evidence of sickness and subsequently entirely 
recovered. The animals were reinoculated two, three, and eight months 
later with one-twentieth to one-fiftieth oese of a pest culture (of which 
one one-hundredth oese represented the fatal dose for a normal guinea 
pig). Seven of the animals remained alive. 
In a large series of rats immunized by various methods, the loss from 
inoculation, with the living attenuated cultures, was 2.3 per cent; with 
the killed agar cultures, 33.3 per cent; with Haffkine’s prophylactic, 38.5 
™ Ztschr. f. Hyg. (1903), 45, 512. 
