48 VARIATIONS IN MORPHOLOGY [CH. iv 



typical of this organism. He examined 72 strains, derived 

 from half a dozen different sources. Out of this number 50 

 strains formed typical colonies but many of them on further 

 cultivation gave rise to atypical colonies, which later, however, 

 reverted to the common type. The remaining 22 strains 

 (30'5 per cent.) all yielded atypical colonies. Many of these 

 colonies bore no resemblance whatever to the common type 

 and showed no tendency to revert to it ; moreover, they 

 differed as much from each other as from the typical colony. 

 (Photographs showing the different appearances are to be 

 seen with the original article.) One strain (No. 160) replated 

 after varying intervals eight times in the course of several 

 months, gave rise to no less than 14 distinct types of colony, 

 all of them atypical. In other respects the organisms proved 

 to be in every case' typical B. coli in pure culture. In his 

 opinion the material from which the organism was isolated 

 considerably influenced the type of colony formed. 



2. The second statement is confirmed by the same in- 

 vestigator who observed that colonies whose appearance was 

 absolutely typical of B. coli, might be composed of different 

 organisms altogether. Many years previously Klein (1899-00) 

 pointed out that it was not safe, from their appearance alone, 

 to regard particular colonies on gelatin as those of B. coli or 

 its varieties. "Such colonies," he remarked "could not without 

 animal experiment be declared not to be the bacillus of 

 pseudo-tuberculosis. Moreover they might be neither B. coli 

 nor its varieties nor the bacillus of pseudo-tuberculosis but 

 some totally different organism." 



W. B. M. Martin (1911) has published photographs showing 

 the different appearances presented by colonies of the gono- 

 coccus grown from the same strain and on the same media. 



3. In the third place it is known that the addition of 

 various substances to a culture medium will modify the 

 character of bacterial colonies growing on it. Thus, Penfold 

 (1911 B, c) observed that the addition to an agar medium of 

 certain carbohydrates developed papillae on the surface of 

 the colonies of many organisms. B. typhosus exhibits this 

 papillae formation on agar containing lactose, dulcite, or 



