STRUCTURE, MOTIONS, REPRODUCTION. 113 



But recently two motile species have been described, and in one of 

 these Micrococcus agilis of Ali-Cohen the presence of flagella has 

 been demonstrated. 



Many of the bacilli and spirilla are actively motile, and the pre- 

 sence of flagella, which has long been suspected, has recently been 

 demonstrated 'for a considerable number of species by Loffler and 

 others. 



It must be remembered that the molecular movement which is 

 common to all minute particles suspended in a fluid is a vibratory 

 motion?'?*- si hi, which does not change the .relative position of the 

 moving particles. This so-called Brownian movement has frequently 

 been mistaken for a vital motion, as has also the movement due to 

 currents in the liquid in which non-motile organisms are suspended. 

 The latter is to be distinguished by the fact that the microorganisms 

 are all carried in one direction. This movement due to a current, in 

 connection with the vibratory Brownian movement, is very deceptive, 

 and it is often hard for a beginner in bacteriological study to con- 

 vince himself that what 'he sees is not a vital movement. But in 

 true vital movements we have progression in different directions, and 

 the individual microorganisms approach and pass each other, often 

 in a most vigorous and active manner, passing entirely across the 

 field of view or changing direction in an abrupt way. Sometimes 

 the motion is slow and deliberate, the bacillus progressing with a to- 

 and-fro motion, as if propelled by a trailing flagellum ; or it may be 

 serpentine when the moving filament is flexible ; or again it is 

 a darting forward motion which is so rapid that the eye can scarcely 

 follow the moving body. The spirilla have a rotary movement as 

 well as a progressive one, and this is often extremely rapid. Some- 

 times bacilli spin around with a rotatory motion, as if they were an- 

 chored fast to a fixed point, as they may be by the flagellum being 

 attached to the slide or cover glass. Frequently, in a pure culture, 

 the individual bacilli may be seen to come to rest, and, after an inter- 

 val of repose, to dart forward again in the most active way. Or we 

 may find, on examining the same culture at different times, that 

 upon one occasion there is no evidence of vital movements, and on 

 another all of the bacilli are actively motile. These differences de- 

 pend upon the age of the culture, temperature conditions, etc. 



Reproduction by binary division is common to all of the bacte- 

 ria, and in many species this is the only mode of reproduction known. 

 When circumstances are favorable for rapid multiplication the indi- 

 vidual cells grow in length, and a constriction occurs in the middle 

 transverse to the long diameter. This becomes deeper, and after a 

 time the cell is completely divided into two equal portions, which 

 again divide in the same way. Separation may be complete, or the 

 8 



