THE BIOLOGY OF BACTERIA 15 



partly understood. Heating the slide or medium accelerates 

 all movement. A fresh supply of oxygen, or indeed the 

 addition of some nutrient substance, like broth, will have 

 the same effect. There are also the somewhat mysterious 

 powers by which cells possess inherent attraction or repulsion 

 for other cells, known as positive and negative chemiotaxis. 

 These powers have been observed in bacteria by Pfeiffer and 

 Ali-Cohen. 



The essential condition in the motile bacilli is the presence 

 of flagella? These cilia, or hairy processes, project from 



BACILLI, SHOWING FLAGELLA 



the sides or from the ends of the rod, and are freely motile 

 and elastic. Sometimes only one or two terminal flagella 

 are present ; in other cases, like the bacillus of typhoid 

 fever, five to twenty may occur all round the body of the 

 bacillus, varying in length and size, sometimes being of 

 greater length even than the bacillus itself. It is not yet 

 established as to whether these vibratile cilia are prolonga- 

 tions of capsule only, or whether they contain something of 



1 A flagellum is a hair-like process arising from the poles or sides of the 

 bacillus. It must not be confused with a filament, which is a thread-like growth 

 of the bacillus itself. 



