UNJVElt 



CLASSiriCATION OF BASSQSjkiSC^ 189 



bacterium and a bacillus, being a question of 

 length. Observers differed as to whether a rod 

 of a certain length ought to be considered a bac- 

 terium or a bacillus. To meet this difficulty a rough- 

 and-ready rule was suggested, viz., that a rod less 

 than twice its breadth in length should be considered 

 as a bacterium, and otherwise a bacillus. But this 

 purely arbitrary division was inadequate, from the fact 

 that a rod at one stage of its growth or under certain 



FIG. 67. BACTERIUM PNEUMONIA CROUPOS.^, X 1500 (after Zopf). 



conditions might, as far as length went, truly be a 

 bacterium, and under other circumstances be of 

 such a length as to entitle its being considered a 

 bacillus. We avoid such confusion if we follow 

 Zopf, and acknowledge as a difference between a 

 bacterium and a bacillus the presence or absence 

 of that form of spore-formation now distinguished as 

 endogenous spore-formation. We can then most 

 conveniently retain this generic term, to include that 

 group of rod-forms in which this spore-formation is 

 as yet unknown ; moreover, we shall find that by so 



