BACTERIUM AN THE A CIS. 



513 



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. 



GROWTH ON AGAR-AGAR. Colonies of this organ- 

 ism, as seen upon agar-agar, present a very typical 

 appearance, from which they have been likened unto 

 the head of Medusa. From a central point which is 



FIG. 84. 



Colony of bacterium anthracis on agar-agar. 



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. 84.) 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. 82.) These bundles 

 twist and cross in all directions, and eventually dis- 



33 



