MORPHOLOGY AND CLASSIFICATION 151 



of flagella. A further serious objection is due to the lack of 

 stability in the character selected to distinguish the ganera. 

 The flagella may disappear from bacteria ordinarily possessing 

 them as a result of changes in environment and may be again 

 made to appear by reversing the conditions.^ Furthermore 

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

 respect to other characters, morphological and physiological, 



■V' 



V, 



Fig. 65. — Bacilli of various forms. 



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. 



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

 several thousand different kinds having been described. They 

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

 60JU to 0.2/i. The very large ones are non-pathogenic species. 



fi-yV-') . CSZ) CMD 



a b c 



Fig. 66. — Sporulation. a. First stage showing sporogenic granules; b, incomplete 

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



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 

 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 at- 

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



