THE VACCINE OF SPLENIC FEVEK. 237 



ture, therefore, and in contact with pure air, we can 

 maintain a culture of filamentous parasites of splenic 

 fever, deprived of all germs. In some weeks the crop 

 dies that is to say, when this culture is sown in fresh 

 broth the sterility of the broth remains complete. But 

 during the preceding days life exists in the cultivating 

 liquid. If after two, four, six, or eight days of ex- 

 posure to the air and to heat, the contagium is tried 

 upon animals, its virulence is found to be continually 

 changing with the time of its exposure to the air, 

 and, consequently, it represents a series of attenuated 

 contagia. From the moment when the formation of 

 the spores of the splenic fever bacillus is prevented, 

 all becomes substantially the same as in the case of 

 the microbe of fowl cholera. Moreover, as in the 

 cholera microbe, each of these states of attenuated 

 virulence can be reproduced by cultivation. Finally, 

 splenic fever not being recurrent, each of these splenic 

 fever microbes constitutes a vaccine for the more 

 virulent microbe. 



In order to apportion the virulence of the vaccine 

 to the species it is desired to vaccinate, it must be 

 tried on a certain number of individuals of the same 

 species. If some vaccinated animals are inoculated 

 with the virulent virus, and none of them perish, the 

 vaccine is good. Among individuals of the same 

 species, however, the difference of receptivity is in 

 general great enough to make it prudent, and even 



