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Morphological Studies in the Life-Histories of Bacteria. 

 By Edward C. Hort, F.B.C.P. Edin. 

 (Communicated by J. Bretland Farmer, F.B.S. Beceived March 2, 1917.) 



(Plates 16-20.) 



It has of course long been recognised that changes in the cultural environ- 

 ment of bacteria may sometimes be followed by corresponding morphological 

 adaptations. By many students such changes in form have been summarily 

 dismissed on the facile theory that cultural contamination has occurred. 

 The rigid precautions taken in these experiments to exclude such accident, 

 and the fact that gemmation took place under observation on the warm 

 stage, dispose of the contamination theory. , 



By other students aberrant morphological types of bacteria are frequently 

 put aside on account of their supposed involutionary nature. The term 

 " involution form " may perhaps be legitimately applied to the bizarre 

 deformities seen in dying or dead individuals in old, or otherwise unsuitable, 

 media. But to apply the term to young, freely growing, freely dividing 

 organisms under the optimum cultural conditions employed throughout in 

 these experiments would be hardly reasonable. 



By still other observers the morphological changes which may follow 

 alteration of bacterial environment have in the past been looked upon as 

 genuine mutation phenomena, and by some authorities the term polymorphic, 

 or pleomorphic, is inaccurately restricted to such alleged examples of 

 mutation. These latter writers deny the occurrence of bacterial mutation, 

 and dogmatically assert that pleomorphism — in the limited sense above 

 defined — is unknown amongst the bacteria, heedless of the fact that the 

 genuine type of pleomorphism exhibited, for example, by the protozoa and 

 the parasitic fungi in the orderly sequence of the manifold cycles of their 

 complex life-histories, would equally apply to the bacteria, once it was 

 proved that they too can reproduce themselves in other ways than by simple 

 transverse binary fission into equal parts. 



Finally, it is often assumed that, because bacteria appear to breed true to 

 type — that is, to laboratory type — in standardised laboratory cultures, simple 

 transverse binary fission is the sole method of reproduction under natural 

 saprophytic and parasitic conditions which are not, and never can be, 

 standardised. 



As, however, will appear, evidence of complex bacterial life-cycles is 

 constantly before us, even in ordinary standardised laboratory media, though 

 it may, and often does, require persistent looking for. 



