INTRODUCTION 19 



rapidly separate themselves from the parent threads (see 

 Ellis (5)). 



Another form of pleomorphism is observed in Crenothrix 

 polyspora, another of the iron bacteria. There are occasions 

 when this organism departs from its normal structure. The 

 cells inside the sheath which envelops them break loose and 

 divide up into minute round fragments, and in that state con- 

 tinue an existence which is totally different from that normal 

 to this organism, so much so that seen separately the two 

 forms would not be assigned to the same species. 



The evidence points to the occurrence of pleomorphism, 

 not as a normal, or perhaps even a frequent phenomenon 

 in the life-histories of the sulphur bacteria, but rather as one 

 likely to occur with great frequency if certain conditions pre- 

 vail, or arise with any degree of frequency. If they do not 

 arise then it is probable that the organism remains unimorphic 

 indefinitely. The possibility, however, of its appearance must 

 be taken into account in framing a system of classification of 

 the sulphur bacteria. 



