BACTERIUM ANTHRACIS. 493 



and the same series of changes will be repeated until 

 the surroundings become again gradually unfavorable 

 to development, when spore-formation again takes place. 

 Spore-formation occurs only at temperatures ranging 

 from 18 to 43 C., 37.5 C. being the optimum. 

 Under 12 C. they are not formed. This organism 

 does not form spores in the tissues of the living 

 animal, its usual condition at this time being that of 

 short rods ; occasionally, however, somewhat longer 

 forms may be seen. 



The bacterium of anthrax is not motile. 



FIG. 85. 



Colony of bacterium antbracis on agar-agar. 



Colonies of this organism, as seen upon agar-agar, 

 present a typical appearance, from which they have 

 been likened unto the head of Medusa. From a central 

 point which is more or less dense, consisting of a felt-like 

 mass of long threads irregularly matted together, the 

 growth continues outward upon the surface of the agar- 

 agar (Fig. 85.) It is made up of wavy bundles in 

 which the threads are seen to lie parallel or are twisted 

 in strands like those of a rope ; sometimes they have a 

 plaited arrangement. (See Fig. 83.) These bundles 

 twist and cross in all directions, and eventually dis- 



