BACILLUS ANTHRACIS. 495 



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 bacillus 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. 94. 



Colony of baeiUus 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. 94.) 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. 92.) These bundles 

 twist and cross in all directions, and eventually dis- 



