144 SUMMARY [CH. x 



amongst bacteria. In other words, a study of such variations 

 in a given case may afford valuable and trustworthy informa- 

 tion as to the source from which the particular strain of 

 organisms has been derived. 



This subject would repay further investigation. One 

 or two instances may be given here to demonstrate its im- 

 portance. 



Rosenow (1912-13) found that the ordinary streptococcus 

 pyogenes, if grown in unheated milk, became modified in its 

 morphology, its cultural properties and its virulence. He had 

 previously isolated from several cases of epidemic sore throat 

 a streptococcus which possessed precisely similar modifications 

 in character. The epidemic had been recognised as "milk- 

 borne" but, had its origin been in doubt, the unusual char- 

 acters of the organism concerned would obviously have pro- 

 vided a clue. 



Ohlmacher (1902) isolated branching filamentous forms of 

 B. coli from the heart's blood in a case of septicaemia. He 

 quotes various observations to the effect that residence in the 

 biliary passages develops this unusual morphology in B. coli, 

 and he therefore considers that the original source of the 

 systemic infection in this case was in the region of the gall 

 bladder or bile ducts. 



Moreover the degree to which the modifications persist on 

 subculture is a measure of the time during which the organism 

 was subjected to the modifying influence. This brings one to 

 the second question, the stability of the variations produced. 



Remarkable differences are to be observed in the degree 

 of permanence exhibited by a variation in different cases and 

 it is difficult to decide upon what factors these differences 

 depend. 



We have already referred to the fact that variations may 

 be either "spontaneous" in character or "impressed" upon 

 the organism by external agencies. Spontaneous variations 

 may be of several kinds and the nature of the variation may 

 itself decide its degree of permanence. 



1. Some variations represent an early stage in the life 

 history or are due to imperfect development, and are seen 



