26 BACTERIOLOGY. 



tufts at the cell poles, or distributed over the 

 whole cell body. Such emphasis is provisional, 

 indeed premature, since little is yet known 

 about the conditions of cilia formation. M. pro- 

 digiosus, for example, is usually non-motile, but 

 is occasionally motile by the aid of flagella ; 

 the cholera germ, which is actively motile in 

 ordinary bouillon, becomes motionless in bouil- 

 lon to which sugar has been added. 



Par more important as an aid to systematic 

 classification are the facts afforded by spore-for- 

 mation. In multiplication by simple division 

 a complete rearrangement of the cell-contents 

 takes place, and the granules in the protoplasm 

 exhibit a much more definite grouping. At- 

 tention has been called elsewhere to this obser- 

 vation as showing an analogy with the arrange- 

 ment of nuclear thread-work during mitotic 

 division. In such a case of simple cell division, 

 both daughter cells serve to perpetuate the 

 species. In other cases, however, this method 

 of reproduction does not suffice for the preser- 

 vation of the species and the ordinary vegetative 

 cells die in great numbers under the natural or 

 artificial conditions to which they happen to be 

 subjected, such as light, heat, dryness, chemi- 

 cal influences and competition with microbes 

 of their own or other species. In this event, 

 certain cells that are more resistant take charge 



