28 FORM AND DIMENSIONS 



29. Mutability of Form. 



The question as to whether any given species of bacterium assumes only one 

 of the forms of growth already described, or has the power of appearing in 

 various shapes, is one of wide interest. A reliable answer could, however, only 

 be given when the necessary appliances for studying the separate species in 

 the form of pure cultures, i.e. cultures practised upon an isolated single cell 

 protected from subsequent contamination by other organisms, had been invented. 

 This possibility was first achieved early in the " eighties," since which time the 

 investigation of this question has proceeded with energy, and the results have 

 shown that the theory of uniformity of growth (monomorphism) is untenable for 

 bacteria, and must give place to the theory of multiformity (pleomorphism). 



The remark made in a previous paragraph that the names bacillus, coccus, 

 &c., simply indicate certain forms of growth, will now first become perfectly 

 clear. When, in future pages, one or other of the various bacteria is qualified by 

 the generic name Bacillus (for instance, Bacillus urece), it is not meant that the 

 said species appears only in the form of rods. On the conti-ary, 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 arthrosporic bacteria, whilst, conversely, the kinds capable of 

 terming endogenous spores are mostly endowed with a smaller number of 

 mutation forms. This difference will be reverted to in a subsequent chapter. 



The inciting cause of mutability is generally external, depending 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 exhaustively 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 require a not unimportant supple- 

 mentary 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 Nageli the name of involution forms. They do not enter into the series of 



