STRUCTURE, MOTIONS, REPRODUCTION. 119 



The cell membrane in certain species appears to be very flexible, 

 as may be seen in those which have a sinuous motion. It is not 

 easily recognized under the microscope, but by the use of reagents 

 which cause the protoplasm to contract may be demonstrated e.g., 

 by iodine solution. Outside of the true cell membrane a gelatinous 

 envelope so-called capsule is sometimes seen. This may perhaps 

 be, as claimed by some authors, nothing more than a jelly-like thick- 

 ening of the outer layers of the cell wall. This jelly-like material 

 causes the cells to adhere to each other, forming zooglcea masses. 

 In some cases the growth upon the surface of a culture medium is 

 extremely viscid, and may be drawn out into long threads when 

 touched with a platinum needle, owing to the gelatinous intercellular 

 substance by which the cells are surrounded. 



There is but little more to be said of the structure of these minute 

 organisms, except to mention the fact that the motile species are 

 provided with slender, whip-like appendages called flagella. The 

 micrococci in general are not endowed with the power of executing 

 spontaneous movements, and they are not provided with flagella. 

 Bat 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 in situ, 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 



