MORPHOLOGY AND CLASSIFICATION 145 



made to appear by reversing the conditions. 1 Furthermore 

 in some groups of bacteria which seem to be closely related in 

 respect to other characters, morphological and physiological, 

 both motile and non-motile forms occur. On the whole the pres- 

 ence or absence of flagella would seem to be too fragile a character 

 to serve as a sole distinction between genera among the rod- 

 shaped bacteria. 



FIG. 63. Bacilli of various forms. 



The different species of rod-shaped bacteria are very numerous, 

 several thousand different kinds having been described. They 

 vary in width from 4/1 to. o.i or probably less, and in length from 

 from6o,to o.2//. The very large ones are non-pathogenic species. 

 The form is ordinarily that of a straight cylinder of equal caliber 

 throughout its length. Certain slightly curved forms are never- 

 theless included in the family, although they may perhaps be 

 regarded as intermediate between the bacteriaceae and the 



CMD CMD 



FIG. 64. Sporulation. a, First stage showing sporogenic granules; b, incomplete 

 spore; c, fully developed spore. (After Novy.) 



spirillaceae. Some of the rod-shaped bacteria are of uneven 

 caliber, especially when growing under unfavorable conditions or 

 when spores are produced. The ends of the rod may be pointed, 

 rounded, square-cut or concave. The bacteria may remain 

 attached after cell-division, forming groups of two, diplo-bacillus , 

 or many cells remain attached, to form long threads, strepto- 

 bacillus. Endospore formation occurs almost exclusively in 



1 Passini: Zts.f. Hyg., 1905, XLIX, pp. 135-160. 



