FORM AND STRUCTURE OF BACTERIA 27 



attenuate the bacteria as far as their ability goes to produce 

 fermentation, putrefaction, disease, etc. 



Another theory the theory of pleogony was also for- 

 merly held by certain bacteriologists. This theory dealt with 



FIG. 20. Involution forms. Bact. tuberculosis, two months' old culture. After 

 Schmidt and Weis. 



the development of new physiological characteristics among 

 bacteria. The typical, normal, and physiologically active 

 bacterial cell will, however, always return when the neces- 

 sary conditions are provided. The passage of some bacteria 

 through animals causes a return to the normal virulence and 

 typical form of the bacterium. 



Dimensions of Bacterial Cells. The size varies with the 

 species. In most instances it is fairly constant within the 

 limit of the same species. In some species the size of the 



