88 Manual of Veterinary 3Iicrobiology. 



and raore remote from tlie establishment of the cul- 

 ture, we obtain generations of. progressively decreas- 

 ing virulence, a series of viruses less and less power- 

 ful, the special activity of which is preserved when 

 we exclude tliem from contact with the air or when 

 we rejuvenate them without intermission by cultures 

 made at very short intervals. Attenuation is here, 

 therefore, hereditary through successive generations. 

 Those of most feeble virulence constitute vaccines 

 against more virulent cultures. 



Cultures of the germ of rouget undergo changes 

 similar to those of fowl cholera. 



As to cultures of the charbon bacillus, they are 

 infinitely more resisting to the destructive influence 

 of the atmosphere. This is because they contain 

 spores, and when we wish to attenuate them in con- 

 tact with the air it is necessary to begin by prevent- 

 ing sporulation. Pasteur has attained this end by 

 cultivating charbon at a temperature of 42° to 43°. 

 At this temperature, the culture exposed to contact 

 with the air, rapidly loses its virulence ; it ceases to be 

 fatal first for the larger animals, then for small adults, 

 finally, for small animals only a few days old.. The 

 bacteridium itself perishes much more slowly. Now, 

 each degree of virulence can be perpetuated by cul- 

 tivating at 42° to 43° the difierent varieties obtained, 

 each of these varieties transmitting its special viru- 

 lence to its descendants. If they are returned to 37° 

 they form spores possessing in embryo the special 

 pathogenic activity of the attenuated bacilli from 

 which they originate, and susceptible of transmitting 

 the latter to new generations cultivated at 37°. 



According to M. Chauveau,the attenuation of the 



