28 SPECIAL' CHARACTERS 



medium. Fortunately, however, foodstuff is used up 

 in the course of multiplication and the waste products 

 of nutritional activity accumulate so that the enor- 

 mous growth of bacteria is limited. Bacteria can no 

 better live in the presence of their excretions than 

 can animals. 



SPECIAL CHARACTERS. 



The cell sometimes surrounds itself by an envelope 

 or capsule outside its natural wall, and this is taken 

 advantage of in identification. It is particularly well 

 developed on bacteria when in or lately removed from 

 animal tissues upon which they have been growing. 

 The exact function or importance of these capsules is 

 not known. 



Some bacteria are able to move from place to place 

 in a fluid medium, and are called, therefore, motile. 

 This is due to the presence of extremely fine filament- 

 ous extensions from the cell wall, which upon micro- 

 scopic examination look like wavy hairs. These are 

 called flagella (sing., flagellurti) . They are arranged 

 either at one end, both ends, or around the whole sur- 

 face of the cell. They propel the bacterium by a quick 

 waving or lashing motion. 



When bacteria are subjected to conditions unfavor- 

 able for their life they undergo various changes of 

 size and shape, none of which are very characteristic 

 except the so-called spore formation. By this is meant 

 the concentration of the vital powers and some of the 

 physical constituents of the bacterial cell within a 

 very small, homogeneous, highly light-refractive body 

 which is resistant to deleterious agencies and which 



