28 MICROBIOLOGY 



of their method of reproduction, unlimited possibility. Prac- 

 tically, however, in experimental work the results are quite 

 different. It has been pointed out by many investigators 

 that in cultures in bouillon, for instance, the number of indi- 

 viduals that will multiply becomes very much reduced in 

 number after a limited time. It is known that the rapidity 

 of death of bacteria in cultures depends upon the nature and 

 quantity of the medium as well as upon the vital forces of 

 the individual organisms that differ with the species. Some 

 cultures of bacteria die in a few days while others will live 

 for weeks or even months. The same variation holds when 

 these bacteria are put in extraneous substances such as in 

 the soil or water. 



Involution forms. When bacteria are cultivated under 

 somewhat unfavorable conditions, or when grown for a long 

 time on artificial media, without being transferred, there 

 frequently occurs a variation in the form and size of the 

 organism. These are referred to as involution or degenera- 

 tion forms. Such forms have been described most frequently 

 in cases of diphtheria. They are, however, now and then 

 detected in cultures of practically all species. With certain 

 organisms, such as the bacterium of tuberculosis 1 and diph- 

 theria 2 , branched forms have been described. These, how- 

 ever, are to be differentiated from the involution forms. 



Pleomorphism. The early observers of bacteria did not 

 recognize distinct genera and species among them. Some 

 of those observers recognized a pleomorphism or mutability 

 of shape. For instance they did not believe that a micro- 

 coccus necessarily always remained a micrococcus but that 

 it could change into a bacillus, a vibrio, and later return to 

 the spherical form of the micrococcus. Pleomorphism is no 

 longer recognized in bacteriology. The subject matter of 

 the whole discussion by the earlier writers may be summed 

 up in pleogeny or mutability of function and pleomorphism 

 or mutability of shape. We now recognize the definite taxo- 

 nomic value of such terms as micrococcus, bacillus, spiril- 

 lum, and the like. 



1 Wolbach and Ernst, Jour, of Med. Research, Vol. X (1903) 

 p. 313. 2 Hill. Annual Rept. of the Boston Board of Health, 1901. 



