92 FRESH-WATER BIOLOGY 



nuclear substance of higher cells. It has been held by different 

 observers that a bacterial cell is to be compared either to a free 

 nucleus or to an unnucleated mass of cytoplasm, but these views 

 have now been practically abandoned. It seems to be clear from 

 the researches of recent investigators that the chromatin substance 

 instead of being gathered together in a definite nucleus, as in the 

 cells of most higher forms of life, is fragmented and distributed 

 irregularly through the body of the cell. The bacterial chromatin 

 is usually present in great abundance, varies in amount and in 

 position in different kinds of bacteria and occurs most frequently 

 in a finely-divided condition. Not only are particles of chromatin 

 scattered through the cell, but other granules that react to stains 

 in special ways are present in the cell substance, particularly in 

 certain species. The physiological significance of these so-called 

 metachromatic granules, as they occur for example in the diphtheria 

 bacillus, is unknown, but it seems probable that they are to be 

 looked upon as reserve food substances. 



Many forms of bacteria show independent movement, distinct 

 from the oscillating or trembling movement exhibited by all minute 

 particles suspended in water and known as the Brownian movement. 

 The power of motility depends upon the possession of long, fragile, 

 filamentous appendages termed flagella. In the case of certain 

 large spirilla, flagella can be seen on the living, unstained cell, but 

 ordinarily special methods of staining must be applied to demon- 

 strate their presence. The position of the flagella on the cell body 

 differs in different species. Some species possess a single flagellum 

 at one pole, as is the case with the cholera spirillum ; others have a 

 flagellum at either pole; others have polar tufts of flagella; and 

 still others possess flagella attached to the sides as well as the 

 ends of the cell (typhoid bacillus) (Fig. 30). In certain nonmotile 

 bacteria, such as the anthrax bacillus, no flagella have been observed. 



Under certain conditions some bacteria pass from the ordinary 

 or vegetative stage into a highly resistant state, known as a spore 

 or endospore. The spores of bacteria are approximately spherical or 

 oval, are stained with great difficulty with the ordinary aniline 

 dyes and resist destructive agencies, such as heat and chemical 

 disinfectants, much better than the vegetative forms from which 



