658 CONDITIONS AFFECTING THE DEATH OF FUNGI. 



Return of 

 attenuated 

 bacilli to 

 virulent 

 bacilli. 



guinea-pigs show almost no reaction when inoculated ; 

 at a somewhat earlier period, guinea-pigs, but not 

 rabbits, are killed. From the tenth to the twenty-fourth 

 day we obtain the so-called " mouse anthrax," that is to 

 say, a cultivation of bacilli which is only able to kill 

 mice. These attenuated bacilli behave, according to 

 Koch, somewhat differently in the body of the mouse 

 than the virulent bacilli, the capillaries of the lung be- 

 coming filled with extremely long pseudo-threads in a 

 manner which is not observed in the case of ordinary 

 anthrax. 



The first and second vaccines prepared by Pasteur for 

 the protective inoculation of sheep are treated for twelve 

 and twenty-four days respectively in the manner above 

 described. 



According to Pasteur's view the oxygen of the air 

 is the chief active agent in this process of attenua- 

 tion ; while Koch regards the temperature as the most 

 important factor, its effect being perhaps also aided 

 by certain products of tissue change of the bacilli. The 

 results of attenuation experiments at varying high tem- 

 peratures show most distinctly that the effect depends 

 almost exclusively on the temperature, and is in fact the 

 result of the action of a certain temperature, as well as 

 of the length of time to which the material is exposed to 

 it. 



In the case of anthrax bacilli attenuated in this 

 manner, researches have been made as to whether they 

 regain their former virulence when cultivated under nor- 

 mal conditions, or whether they permanently retain the 

 attenuated virulence when once it is acquired. On the 

 whole the experiments have not given an uniform result, 

 and are as yet too few in number to enable us to deduce 

 any law which accounts for the return of the virulence 

 in some cases, and the absence of such return in others. 

 The virulence appears to be regained most readily and 

 most regularly where the attenuation has been brought 

 about, according to Toussaint's method, by the short 

 action of high temperatures ; the bacilli which have been 

 attenuated by ten minutes' exposure to 55 C. regain 



