GENERAL CHARACTERISTICS OF BACTERIA 31 



sometimes distributed over the entire body of the cell, or they may 

 form a tuft at one end of the rod, or only one polar flagellum is found. 

 The polar flagella appear on the bacteria shortly before division. 

 The flagella are believed to be formed of the ectoplasma, and proba- 

 bly have the property of protrusion and retraction. So far as we 

 know, the flagella are the only means of locomotion possessed by the 

 bacteria. They are not readily stained, special staining agents being 

 required for this purpose. The envelope of the bacteria, which usually 

 remains nmstained with the ordinary dyes, then becomes colored and 

 more distinctly visible than is commonly the case. Occasionally, 

 however, some portion of the envelope remains unstained, when the 

 flagella present the appearance of being detached from the bodyfof 

 the bacteria by a narrow zone. In cultures of richly flagellated bacteria 

 peculiar pleated masses sometimes are observed, consisting of flagella 

 which have been detached and then matted together. Bacteria may 

 lose their power of producing flagella for a series of generations. Whether 

 their^ power be permanently lost or not we do not know. 



FIG. 15 Fro ifi 



Jfc; . 





rf? 

 >% v 



Bacilli showing one polar flagellum. Bacilli showing multiple flagella. 



REPRODUCTION AMONG THE LOWER BACTERIA. When a bacterial 

 cell is placed in favorable surroundings for development it multiplies, 

 as a rule, by simple division. When such development is in progress a 

 single cell will be seen to elongate, in the case of spherical bacteria only 

 slightly, and in the rod-shaped organisms considerably in one direction. 

 Over the centre of the long axis thus formed will appear a slight inden- 

 tation in the outer envelope of the cell; this indentation increases in 

 extent until there exists eventually two individuals. As a rule, the cells 

 separate from one another soon after division, but occasionally they 

 remain together for a time, forming pairs and chains. Under certain 

 conditions of nutrition long threads or filaments are formed, which, 

 however, when put in contact with new food, break up into fragments. 

 At times, when the culture media are exhausted or nearly so, the bacilli 

 and spirilla will be found to go on dividing, with little or no increase 



