138 SCHIZOMYCETES 



(6) The medium became toxic from the accumula- 

 tion of metabolic products. 



(c) The environment became unfavourable; e. g., 

 change of temperature. 



This is not altogether correct; e. g., the temperature 

 at which spores are best formed is constant for each 

 bacterium, but varies with different species; again, 

 aerobes require oxygen for sporulation, but anaerobes 

 will not spore in its presence. 



(A) Arthrogenous : Noted only in the micrococci. 

 One complete element resulting from ordinary fission 

 becomes differentiated for the purpose, enlarges, and 

 develops a dense cell wall. One or more of the cells in a 

 series may undergo this alteration. 



This process is probably not real spore formation, 

 but merely relative increase of resistance. . These so- 

 called arthrospores have never been observed to ''ger- 

 minate,". n<?r is their resistance very marked, as they 

 fail to initiate new cultures, after having been exposed 

 to a temperature of 80 C. for ten minutes. 



(B) Endogenous: The cell protoplasm becomes dif- 

 ferentiated and condensed into a spherical or oval 

 mass (very rarely cylindrical) . After further contrac- 

 tion the outer layers of the mass become still more 

 highly differentiated and form a distinct spore mem- 

 brane, and the spore itself is now highly refractile. 

 It has been suggested, and apparently on good grounds, 

 that the spore membrane consists of two layers, the 

 exosporium and the endosporium. Each cell forms 

 one spore only, usually in the middle, occasionally at 

 one end (some exceptions, however, are recorded; e. g., 

 B. inflatus). The shape of the parent cell may be un- 

 altered, as in the anthrax bacillus, or altered, as in the 

 tetanus bacillus, and these points serve as the basis 

 for a classification of spore-bearing bacilli, as follows: 



(A) Cell body of the parent bacillus unaltered in 

 shape (Fig. 91, a). 



