FORM AND STRUCTURE OF BACTERIA 25 



The cocci do not seem to vary perceptibly under any condi- 

 tions. It is possible to cause a regeneration of these " involu- 

 tion forms " in certain species when favorable conditions are 

 provided. // is not possible to cause any permanent change in 

 the morphology of a bacterium and thus originate a new species 

 of bacteria. For example, it is not possible to cause a bacillus 

 to change into a coccus and vice versa. This idea of pleo- 

 morphism was held for years by certain bacteriologists (Zopf . 

 et al.). They explained the origin of new species by means of 

 this phenomenon. Pleomorphism does not exist in its true 

 sense; that is, there is no permanent change in form of a 

 definite species of bacteria. The bacteria if- modified from 

 their normal shape will, when supplied with the requisite 

 conditions of temperature, food, etc., as before stated, return 

 to the normal. There may be a change in the characteristic 

 form of growth of some bacteria under certain conditions. 

 Bacterial cells may form themselves into pellicles, zooglcea, 

 chains, filaments, etc., but the type of cell remains the same. 

 Chloride of lithium, and certain other chemicals, when placed 

 in a medium containing bacteria, will cause a modification of 

 their shape from the normal. 



In certain species of bacteria the individual cells are more 

 prone to change their shape under varying conditions than 

 others, e.g. Bact. diphtherias, Bact. tuberculosis, bacteria in 

 the root tubercles of clover, etc. (Figs. 19 and 20). Various 

 species of bacteria produce " involution forms " under differ- 

 ent conditions. For example, the acetic, lactic, butyric acid 



