CHAPTER IX. 

 BACILLUS (Desmobacterium, Cohn). 



General Characters. Bacilli are cylindrical or rod shaped 

 bacteria, which are rounded or square-cut at their extremi- 

 ties ; they are longer in proportion to their thickness than 

 bacterium termo, and divide by fission, forming straight, 

 curved, or zigzag chains of two, four, six, or more elements. 

 Many species of bacilli in suitable nourishing material grow 

 by repeated division into longer or shorter chains of bacillus 

 filaments or leptothrix. These appear straight or wavy 

 and twisted, isolated or in bundles ; and although in the fresh 

 condition they appear of a homogeneous aspect, when suitably 

 prepared, as by drying and staining with aniline dyes, they 

 show themselves composed of shorter or longer cubical, 

 cylindrical, or rod-shaped protoplasmic elements, contained 

 in linear series within a general hyaline sheath : between 

 many of the elements is a fine transverse septum. The 

 isolated bacilli are likewise composed of a membrane and 

 protoplasmic contents. These latter appear homogeneous or 

 finely granular, and when stained with aniline, absorb the dye 

 very easily and retain it better and longer than the sheath. 

 According to the stage and the rapidity of their growth, the 

 bacilli vary much in length ; this is the case not only with 



