CYTOMORPHOSIS IN BACTERIA 147 



new mode of growth. But we have every evidence that in other 

 organisms modes of reproduction other than simple vegetative di- 

 vision or budding are also dependent upon the age of the culture. 

 As Child states, "there is good reason to believe that algae and 

 fungi may undergo senescence and rejuvenescence like the lower 

 animals, and that the different forms of reproduction are character- 

 istic of different stages of the life cycle" (i.e., phases of growth). 

 It is quite possible then that bacteria do develop during the death 

 phase some sorts of reproductive bodies that give rise to particular 

 forms of resting cells; that there does occur, as Mellon claims, 

 some sort of a reorganization process. But since we know that in 

 higher organisms along with these reproductive processes there occur 

 retrogressive changes in the vegetative cells, we are justified in de- 

 manding conclusive proof that these unusual cells found in the death 

 phase are really reproductive cells and not simply degenerating 

 cells. The burden of proof of complex life cycles in bacteria must 

 be placed upon the shoulders of those men who have declared that 

 such life cycles exist. So far such proof has not been brought for- 

 ward. 



In connection with these theories of pleomorphism, it should 

 also be pointed out that every study of life cycles of bacteria should 

 be accompanied by observations of the rate of growth and a de- 

 termination of the phase of growth in which each type of cell 

 change occurs. The age of the culture is of paramount importance 

 in interpreting the nature of the cell variations which are observed, 

 and by this I mean physiologic age rather than age in hours. It is 

 not sufficient to state that a culture is so many hours or days old, 

 for the rate of growth may be varied by many factors. As has 

 been shown in the earlier part of this work, during the growth 

 phase cells may change in size within one or two hours to such a 

 degree that they would by ordinary standards not be recognized 

 as the same. One cannot, therefore, safely guess at the phase of 

 growth; it must be determined by measurement. Many of the 

 morphologic variations which have hitherto been described, par- 

 ticularly those indicating a possible mutation in the organisms studied, 

 have undoubtedly been not true permanent variations but the re- 

 sult of a comparison by the investigator of cultures which are in 



