14 BIOLOGY AND TECHNIQUE 



all cases to distinguish between actual motility and the so-called Brown- 

 ian or molecular movement which takes place whenever small particles 

 are held in suspension in a fluid. 



Brownian or molecular movement is a phenomenon entirely ex- 

 plained by the physical principles of surface tension, and has absolutely 

 no relation to independent motility. It may be seen when particles of 

 carmine or any other insoluble substance are suspended in water, and 

 consists in a rapid to and fro vacillation during which there is actually 

 no permanent change in position of the moving particle except inas- 

 much as this is influenced by currents in the drop. 



The true motility of bacteria, on the other hand, is active motion 

 due to impulses originating in the bacteria themselves, where the actual 

 position of the bacterium in the field is permanently changed. 



The ability to move in this way is, so far as we know, limited almost 

 entirely to the bacilli and spirilla, there being but few instances where 

 members of the coccus group show active motility. In all cases, with 

 the exception of some of the spirochetes, where motility may occasionally 

 be due to an undulating membrane marginally placed along the body, 

 bacterial motility is due to hair-like organs known as flagella. These 

 flagella have rarely been seen during life, and their recognition and study 

 has been made possible only by special staining methods, such as those 

 devised by Loeffler, van Ermengem, Pitt, and others. 



In such stained preparations, the bacterial cell bodies often appear 

 thicker than when ordinary dyes are used, and the flagella apparently 

 are seen to arise from the thickened ectoplasmic zone. 



The flagella are long filaments, averaging in thickness from one-tenth 

 to one-thirtieth that of the bacterial body, which often are delicately 

 waved and undulating, and, judging from the positions in which they 

 become fixed in preparations, move by a wavy or screw-like motion. 

 In length they are subject to much variation, but are supposed to be 

 generally longer in old than in young cultures. Very short flagella have 

 been described only on nitrosomonas, one of the nitrifying bacteria 

 discovered by Winogradsky.^ 



As to the finer structures of flagella, little can be made out except 

 that they possess a higher refractive index than the cell body itself, 

 and that they can be stained only with those dyes which bring 

 clearly into view the supposedly true cytoplasm of the cell. 

 Whether they penetrate this cytoplasmic membrane or whether they 



1 Winogradsky, Arch, des sci. biologiques, St. Petersburg, 1892, I, 1 and 2. 



