CHAPTER IX 



THE SENESCENT FORMS OF THE COLON BACILLUS 



The occurrence of irregular cell types which I have designated 

 as asymmetrical in the preceding chapter seems to be clearly cor- 

 related with death. Now these cells are just those forms which have 

 received the greatest attention from the exponents of life cycles in 

 bacteria, and just those types which have been most the subject 

 of controversy between the monomorphists and pleomorphists; the 

 former, by designating them involution forms, imply an abnormality 

 due to actual or approaching death; the latter believe them to be 

 reproductive cells of one type or another. I do not believe that any 

 data have as yet been brought forward to definitely settle this prob- 

 lem, for it may well be that those factors which lead to the death 

 of some of the cells in a culture may react upon others to cause 

 them to develop into one or another type of resting cell or reproduc- 

 tive body; but their occurrence only in the death phase is strongly 

 presumptive evidence of the orthodox view that they are involution 

 forms, and places squarely upon the pleomorphists the burden of 

 proving that they are otherwise. By naming these irregular types 

 "senescent" forms I express my opinion that the monomorphistic 

 teaching regarding them is correct. But I prefer this term to "in- 

 volution form" because it clearly expresses the analogy between 

 them and the cell changes which occur in old age in multicellular 

 organisms, and because it does not so much imply that they are 

 already actually dead. 



The observations recorded in preceding chapters have for the 

 most part not been continued long enough to include more than the 

 very beginnings of the death phase. In none of them have plate 

 counts been made, so that it is not possible to determine the true 

 rate of death. Moreover, the measures of morphologic variation 

 used do not reveal except roughly the changes in the cells which 

 seem to be most characteristic of the death phase. It was 

 thought desirable therefore to continue these studies with some ob- 



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