BACILLUS ANTHRACIS. 501 



once on animals. The degree of attenuation experi- 

 enced by the cultures grown under these circumstances 

 is determined by tests upon rabbits, guinea-pigs, and 

 mice. The first culture removed may or may not 

 kill rabbits, the most resistant of the three animals 

 used for the test, while it will certainly kill the guinea- 

 pigs and mice ; after another two or three days rabbits 

 will no longer succumb to inoculation with the culture 

 last removed from the incubator, while no diminution 

 will as yet be noticed in its pathogenesis for the other 

 two species. After four to seven days more a culture 

 may be encountered that kills only mice, the guinea- 

 pigs escaping ; while ultimately, if the experiment be 

 continued, a degree of attenuation may be reached in 

 which the organism has not even the power of killing 

 a mouse, though it still retains its vitality. Investiga- 

 tion of these attenuations shows them to possess all 

 the characteristics of enfeebled anthrax bacilli ; they 

 grow slowly and less vigorously when transplanted ; 

 they do not form spores \\'hen exposed to a high tem- 

 perature ; and microscopically they present evidences 

 of degeneration. When introduced beneath the skin 

 of animals they disseminate but slightly beyond the 

 site of inoculation, and do not, as a rule, cause the 

 general septicaemia that occurs in susceptible animals 

 after inoculation with normal cultures of this organism. 

 In the practical employment of these attenuated cult- 

 ures for protective purposes two vaccines are emploj^ed. 

 These were designated by Pasteur as "first" and 

 " second " vaccines. The " first " i.s the one that killed 

 only the mice in the preliminary tests ; while the " sec- 

 ond" is that which killed both mice and guinea-pigs, 

 but failed to kill the rabbit. When larger animals, 



