510 
some of the animals succumbed from the result of the vaccination itself— 
i. e., they died of a true plague infection, hence the limit of safety had 
been passed. Naturally, only cultures which were invariably non-fatal 
for animals could be employed or recommended for use in human beings, 
and with these cultures it has been possible to render immune, by a single 
inoculation and a moderate dose, only from 60 to 80 per cent of the ex- 
perimental animals. 
Therefore, when the announcement was made that, by immunization 
with repeated doses of aggressin, guinea pigs could also be protected 
against plague infection, the importance of this claim was readily ap- 
preciated, and I determined to convince myself of the value of this 
method and at the same time to discover if guinea pigs, perhaps the most 
susceptible of all animals to plague, could not be immunized with cer- 
tain bacterial extracts of the plague bacillus, particularly since the 
method of immunization with animal exudates as it is recommended by 
Bail can not be considered practicable for the inoculation of large 
numbers of people. In this connection it may be mentioned that by 
experimental work I had already found that an extract of the plague 
bacillus obtained in a manner similar to that already employed in this 
laboratory in the manufacture of our cholera prophylactic * from the 
cholera spirillum was not sufficiently potent, since not over 40 per cent 
of the animals inoculated with large doses thereof proved to be fully 
immune. I therefore determined to modify this method and to employ 
such means as. would injure the chemical substances of the plague ba- 
cillus as little as possible during the process of extraction. For this 
purpose a procedure similar to that suggested and used by Wassermann = 
for the purpose of obtaining immunizing substances from typhoid and 
hog-cholera bacilli, and employed in a modified manner by Brieger, 
Bassenge and Meyer,?* was chosen. 
Cultures of a strain of the plague bacillus of the highest obtainable 
virulence were grown upon the surface of large test tubes of agar during 
forty-eight hours at 30° C. The growth was then suspended in distilled 
water, 1 cubic centimeter of water being employed for each 30 mil- 
ligrams of bacteria (a very concentrated suspension). The resulting 
mixture was then placed in a sterilized bottle and fastened upon an 
electrical shaking machine and thoroughly shaken for five days. At the 
end of this time it was removed and heated at 44° to 45° C. for one 
” Strong: American Medicine (1903), 6, 272; Publications Biological Labora- 
tory, Bureau of Government Laboratories (1904), 16, 1; Journal of Infectious 
Diseases (1905), 2, 107. 
2 Wassermann und Citron: Deutsche Medizin. Wehnschr. (1905), 31, 1102. 
* Brieger: Sonderabdruck aus den Verhandlungen des deutschen Kolonialkon- 
gresses (1905), Sektion 2, 182; also Brieger und Meyer, Deutsche Medizin. 
Wehnschr. (1904), 30, 980; Bassenge and Meyer, Centralb. fiir Bakteriol. Abt. 1 
(1904), 36, 332. 
