36 THE SCHIZOMYCETES. 



said species appears only in the form of rods. On the contrary, 

 it may possibly present itself in the form of cocci, threads, &c., 

 and has only received the generic name of Bacillus on account of 

 its generally having this form, and especially when the culture 

 has attained the acme of its development. 



This capacity commonly known as mutability of assuming 

 a variety of shapes is not possessed in an equal degree by all 

 bacteria, and in a few of them it even appears to be altogether 

 lacking. Abundant pleomorphism chiefly prevails in the arthro- 

 sporic bacteria, whilst, conversely, the kinds capable of forming 

 endogenous spores are mostly endowed with a smaller number 

 of mutation forms. This difference will be reverted to in a sub- 

 sequent chapter. 



The inciting cause of mutability is generally external, depend- 

 ing on the conditions to which the culture is subjected. Since 

 these can, to a certain extent, be arbitrarily determined and 

 controlled, a means is thus at hand of exercising a formative 

 interference in the existence of these organisms. Among the 

 morphological forces thus available, two are particularly powerful, 

 viz., the influence of temperature and the composition of the 

 nutrient medium. The former has been elucidated by the studies 

 of E. Chr. Hansen on the acetic bacteria, which will be exhaus- 

 tively discussed in Chapter xxxvii. On the other hand, H. 

 Buchner convincingly demonstrated the influence of the mode of 

 nutrition on the cell form of the hay bacillus (reported in Chapter 

 xvii. ). 



A large number of the species of bacteria noticed in the 

 following paragraphs are pleomorphic, and there will therefore be 

 ample opportunity of becoming acquainted with this phenomenon 

 in all its details and varieties. 



30. Involution Forms. 



The remarks made in the preceding paragraphs with regard to 

 the influence of the composition of the nutrient medium re- 

 quire a not unimportant supplementary explanation. When we 

 arbitrarily bring about an alteration in the form of a cell by 

 modifying the conditions of the culture, a suitability of the 

 medium to the evolution of the species of bacterium in question 

 is presupposed, so that the vitality and reproductive power in 

 the new form of cell thereby produced is preserved. 



This is, however, no longer the case in the modifications of 

 form which result when the medium is exhausted of nutrient 

 materials and consequently enriched with injurious metabolic 

 products. These degenerations of the cell have received from 

 ISTageli the name of involution forms. They do not enter into 

 the series of types already drawn up ( 27), and have no regular 



