CLASSIFICATION AND DESCRIPTION OF SPECIES. 



483 



bacterium 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 other- 

 wise a bacillus. But this purely arbitrary division was inadequate, 

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

 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 (Fig. 197). We should avoid such confusion 

 if we followed Zopf, and acknowledged 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 might then conveniently retain this generic term, to include 

 that group of rod-forms in which this spore-formation is as yet 



FIG. 197. FKIEDLANDER'S PNEUMOCOCCUS, x 1500 (Zopf). 



unknown ; moreover, we should, by so doing, with one or two 

 exceptions, collect together those short rod-forms which appear to 

 link the simple cocci to the spore-bearing rods or bacilli. 



The grouping together of the different species according to the 

 character of the colonies in nutrient gelatine is also of questionable 

 advisability. These characters can hardly be considered to be of 

 sufficient importance, or indeed in many cases to be sufficiently 

 constant, to serve by themselves for this purpose. In many cases 

 a slight variation in the composition of the nutrient medium may 

 considerably affect the appearances of the colonies. At the same 

 time, the appearances are very characteristic of certain species of 

 bacteria, and in some cases the characters of the colonies, together 

 with the characters of the growth in test-tubes, assist us in 

 distinguishing species which are morphologically similar, as in the 

 case of the comma-bacilli of Tinkler and of Koch. 



