THE STORY OF GERM LIFE. 



of the points for distinguishing species. Some 

 species do not form spores, at least under any of 

 the conditions in which they have been studied. 

 Others form them readily in almost any condition, 

 and others again only under special conditions 

 which are adverse to their 

 life. The method of spore 

 formation is always uni- 

 form for any single species. 

 Whatever be the method 

 of the formation of the 

 spore, its purpose in the 

 life of the bacterium is al- 

 ways the same. It serves 

 as a means of keeping the 

 species alive under condi- 

 tions of adversity. Its 

 power of resisting heat or 

 drying enables it to live 

 where the ordinary active 

 forms would be speedily 

 killed. Some of these 

 spores are capable of re- 

 sisting a heat of 180 C. (360 F.) for a short time, 

 and boiling water they can resist for a long time. 

 Such spores when subsequently placed under fa- 

 vourable conditions will germinate and start bac- 

 terial activity anew. 



FIG. 14. Formation of 

 spores in unusual forms 

 (Crenothrix). 



Some species of bacteria have the power of 

 active motion, and may be seen darting rapidly 

 to and fro in the liquid in which they are grow- 

 ing. This motion is produced by flagella which 

 protrude from the body. These flagella (Fig. 15) 



