Morphological Studies in the Life-Histories of Bacteria. 469 



The truth, in fact, appears to be that we have gone astray in this matter, 

 because we have in the past invoked too easily the theories of contamination, 

 of involution forms, and of mutation,* and have forgotten that the natural 

 environment of bacteria, whether as saprophytes or as agents of disease, 

 is in a perpetual state of flux, 



From the morphological observations here recorded, it is clear that bacteria 

 can, and do, reproduce themselves in other ways than by simple binary 

 fission alone, and that the life-cycle in some cases includes an invisible, or 

 almost invisible phase. Our present conception, therefore, of the role played 

 by bacteria, both as saprophytes and as causal agents of disease and its 

 sequels, will have to be profoundly modified. 



According to current bacteriological theory the " lower " bacteria, whether 

 living a parasitic or a saprophytic existence, are unicellular organisms which 

 are only capable — apart, in certain cases, from endospo'rulation of a special 

 type — of reproducing themselves by a simple process of transverse binary 

 fission into two equal parts. 



In previous publications (1) of studies of the life-history of bacteria, 

 I have brought forward evidence which strongly suggested, if it did no more, 

 that the life-cycle of certain of the " lower " bacteria is one of great com- 

 plexity. From this evidence the conclusion was difficult to escape that 

 under parasitic conditions simple binary fission is probably only one of 

 many phases in the bacterial cycle, which includes an invisible, or almost 



* As already stated, the observations here recorded are confined to cultures from 

 single primary colonies or from single individuals, and in consequence these observations 

 only deal with changes in morphology which are not a-ssociated with observed changes 

 in cultural, fermentative and serological reactions. I have, however, noted, especially 

 in the case of the B. Shiga-Kruse and of the B. paratyphosus B, that' profound cultural, 

 " biochemical," and even serological, changes may take place in the case of secondary 

 colonies from pure cultures in some of the plates after prolonged standing and repeated 

 subculturing from acid media. Changes in fermentation reactions of secondary colonies 

 of bacteria have in the past been frequently noted by numerous observers, and the 

 occurrence of a genuine form of bacterial mutation has been invoked to explain these 

 changes. In many such cases the occurrence of morphological types of aberrancy has 

 also been noted, and again the mutation theory has been used in explanation thereof. 

 The most striking of these are the observations of Horrocks (5) in 1911 in the case of 

 the B. typhosus, independently confirmed by Almquist in the present year. In all these 

 observations, however, of changes in morphology, whether associated or not with 

 fermentation and serological changes, the question of such changes representing phases 

 in bacterial life-histories in orderly sequence does not appear till now to have been 

 raised. And this appears to be due to the fact that the morphological changes have 

 not been studied by continuous observation of growth from single organisms on the 

 warm stage, which I have shown to be essential to correct interpretation of alleged 

 mutation phenomena, whether of a morphological or of a " biochemical " nature. 



