I Acquired immunity against micro-organisms 287 

 strong virus into a vaccinated animal is equivalent to the inocula- 

 tion of an attenuated virus. The attenuation, however, instead of 

 being done beforehand in the laboratoiy, is brought about in the 

 l^fttissues of the vaccinated animal " (p. 18). Charriu and Roger ^ up- 

 ^^^iield this view, and the latter offered several new arguments in 

 s support of it. He observed that animals inoculated with pneumo- 

 ll^ftcocci and streptococci grown in the blood serum of vaccinated 

 ■f^^animals, contracted a transient and benign disease merely, whilst 

 f the control animals, inoculated with the same micro-organisms, 



I cultivated in normal serum, always died from generalised in- 

 fection. 

 The discovery of the protective property of serums has thrown a 

 new light upon these experiments. We must now ask ourselves : 

 Does the innocuousness of micro-organisms depend not on the at- 

 tenuation of the virus, but rather on the protective action of the 

 serum itself? When, in the course of my researches on the Gentilly 

 cocco-bacillus, I found that this organism, cultivated in the serum 

 of vaccinated rabbits, became much less pathogenic than when it 

 was grown in the serum of normal rabbits, I set myself to answer 

 this question. Simple filtration through paper was sufficient to rid 

 the organism of the serum in which it had grown. The inoculation 

 of cocco-bacilli thus treated proved at once that their virulence was 

 in no degree modified, and that it was the intervention of the serum 

 that prevented the micro-organism from setting up the rapidly fatal 

 disease. Issaeff ^, who, in my laboratory, carried out the investigation, 

 was able to extend this to the pneumococcus. He obtained agglu- 

 tinated cultures in the serum of vaccinated rabbits, and he compared 

 their activity by injecting them (1) with, and (2) without their culture [302] 

 medium. The difference was very marked. In the first case the 

 infection produced was much slower in its course than in the second. 

 The virulence of the washed pneumococci was found to be the same 

 whether they came from a culture in normal serum or from one in 

 immunised serum. Sanarelli^ obtained the same result with Gama- 

 leia's vibrio. The vibrios when grown in the serum of vaccinated 

 guinea-pigs proved to be very virulent so soon as they were freed 

 from the fluid in which they were grown. Later, similar demon- 



1 Charrin, Compt. rend. Soc. de Uol, Paris, 1890, pp. 203, 332 ; Roger, ibid., ' 

 1890, p. 573, and Rev. gen. d. sc.pures et appliq., Paris, 1891, p. 410. 



2 Ann. de I' Inst. Pasteur, Paris, 1893, t. vn, p. 273. 



3 Ann. de VInst. Pasteur, Paris, 1893, t. vn, p. 231. 



