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 zodgloea 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. 
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 Léffler 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 7 s?tu, 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 microédrganisms 
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 microérganisms 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 
