BACTERIUM ANTHRACIS 559 



roundings are altered, continue in this living, though inactive, 

 condition for a very long time. If again placed under favor- 

 able conditions, each spore will germinate into a mature cell,, 

 and the same series of changes will be repeated until the 

 surroundings become again gradually unfavorable to develop- 

 ment, when spore-formation again takes place. Spore forma- 

 tion 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 



Fig. 98 



Colony of bacterium anthracis on agar-agar, 



that of short rods; occasionally, however, somewhat longer 

 forms may be seen. 



The bacterium of anthrax is not motile. 



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. 98.) 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; 



