509 
have also demonstrated that, by the injection of the aggressin exudates 
into animals, a higher immunity against chicken cholera resulted than 
had been obtained by any other method. 
However, hardly three months ago, Citron,'? working in Wassermann’s 
laboratory, published experiments which showed that with sterilized 
aqueous extracts of the hog-cholera and swine-plague bacillus, he was 
also able to obtain a similar immunity in animals and to produce an 
immune serum which was of equal value with and which acted in a 
manner exactly similar to an anti-aggressin serum. He points out that 
the aggressin immunity is therefore not an immunity sui generis. 
Finally, in the last paper to appear from Hueppe’s Institut (in which 
laboratory all the work of Bail and his associates has been carried on), 
the announcement is made by Hueppe and Kikuchi?’ that successful 
results have been obtained in the immunization of guinea pigs against 
plague infection by repeated inoculations with exudates containing plague 
aggressin. The experiments which are related briefly are very few in 
number, but the authors mention that in this paper it is their intention 
merely to call attention to the priority of the use of this method of 
immunization of animals against plague. 
Up to the present time we were aware of no other reasonably sure 
method of immunizing guinea pigs against plague infection except by 
that one in which the living, attenuated plague bacillus was inoculated. 
Upon this subject I have recently reported in detail in the PrinrepiIne 
JOURNAL OF ScrENCE."® From numerous experiments on animals I had 
found that the living, attenuated plague bacillus caused higher immunity 
than the dead cultures of this organism in any form. However, while 
the method of vaccination was by far the most favorable one for prac- 
ticable immunization, it could not perhaps, be said to be absolutely 
satisfactory from an experimental standpoint, because the single inocula- 
tion of a large dose of an avirulent plague culture, of such attenuation 
that it is never capable of causing the death of an animal even in very 
much larger amounts, does not invariably protect the animal against 
subsequent plague infection. Thus, with monkeys, from 60 to 80 per 
cent of the different series of those vaccinated with such a culture pos- 
sessed high immunity and survived a subsequent inoculation of a multiple 
lethal dose of the virulent pest bacillus, but the remaining 20 to 40 per 
cent were not thoroughly immunized by the single vaccination and suc- 
cumbed to the subsequent injection of the virulent organism. By using 
more virulent cultures for vaccination a higher percentage of animals was 
protected. However, in order thoroughly to immunize a greater propor- 
tion of the monkeys, it was necessary to use cultures of such virulence that 
“Citron: Centralbl. fiir Bakteriol. Orig. (1905), 40, 610. 
*“Hueppe und Kikuchi: Centralbl. fiir Bakteriol. Orig. (1905), 39, 610. 
“Strong: Philippine Journal of Science, (1906), 1, 181. 
