16 A MANUAL OF BACTERIOLOGY 



Rend., clii, 1911, p. 53 ; Prazmowski, Bull. Internal, de VAcad. des 

 Sci. de Cracovie, No. 4, B, April 1913, p. 105 (Bibliog.). 



Classification of the Bacteria 



The classification of the Bacteria is a difficult subject. 

 A somewhat heterogeneous group of organisms has un- 

 doubtedly been described under the term Bacteria, and 

 organisms exist which it is difficult to decide whether 

 they be unicellular or multicellular, whether simple forms 

 or more highly-organised fungi. Moreover, bacterial cells 

 are so minute that only a few broad differences can be 

 observed in the morphology, structure and reproductive 

 processes of different species, and ordinary criteria are 

 therefore not available for the classification of the Bacteria, 

 while the occurrence of variations, known as pleomor- 

 phism, and mutations adds to the difficulty. 



By pleomorphism is meant a variation in the form of 

 an organism during its life-cycle, a rod, for example, 

 giving rise to ovoid, spherical or spiral forms. In a 

 peach-coloured bacterium investigated by Lankester, 

 spherical, rod, filamentous, and spiral forms occurred, 

 and the occurrence of pleomorphism was supported by 

 his work, though it may now be questioned whether he 

 was working with pure cultures. Be that as it may, a 

 certain amount of pleomorphism undoubtedly occurs 

 with some organisms. The cells of the colon, typhoid and 

 plague bacilli, for example, which are typically short 

 rods, may assume the form of ovoids, long rods or even 

 filaments (see also p. 6). 1 



The abnormal forms of cells which are commonly 

 seen in old cultures are termed " involution forms," and 

 are generally regarded as altered, degenerate and decrepit 

 cells. Some of these " involution " forms are. however, 



1 See Dobell, Journ. of Genetics, ii, pp. 201, 325. 



