THE CILIA OF BACTERIA 21 



tion, or a violent trembling of the organism may be the only result 

 of the activity of the cilia, or finally there may be a forward 

 movement unaccompanied by either trembling or rotation. In the 

 rod and spiral bacteria also, we see the same movements, except that 

 the latter do not exhibit a rotatory movement, and in the former 

 this kind of movement is very uncommon. The rotation of an 

 individual belonging to the rod-bacteria is confined to an occasional 

 turn round, without moving forward. When an obstacle is encountered 

 the individuals seem to have no difficulty in changing their direction, 

 and occasionally they may be seen retracing the path by which they 

 have just come. In general the spiral varieties have more rapid 

 movements than the others, and by appropriate cultivation it is 

 possible to increase this rapidity. In the author's experiments with 

 Spirillum giganteum, an increase of rapidity was obtained in the 

 following way : A fresh culture of this species was made every day, 

 the material for each inoculation being taken from the youngest 

 culture, which would thus be 24 hours old. After a month or two 

 of this treatment, the individuals of young cultures of this species 

 were so motile that when examined microscopically in a drop of water 

 the microscope-field presented a blurred appearance as if out of focus. 

 The cilia preparations of such cultures showed about 30 or more cilia 

 at the poles of some of the individuals. Some bacteria are so slow in 

 their movements that it is often difficult to tell whether they are 

 moving or not. If the movement is very slow it is difficult to 

 distinguish it from the Brownian or molecular movement which the 

 smallness of the bacteria subjects them to. An average speed is from 

 3 p. to 6 //, per second. The great differences that exist in the rate of 

 movement is partly due to the variation in the number and strength 

 of the cilia, and also partly to the variable nature of the mucilaginous 

 layer which is_ found covering the cell-wall, by which it has been 

 produced. The greater the amount of mucilage the slower the move- 

 ment, and of course the converse also holds true. 



Species normally motile often go through a portion or even the 

 whole of their life-history without exhibiting motion of any kind. 



Some bacteria are motile during the whole, whilst others are motile 

 for only a small portion of their lives. In the case of those species 

 which form spores, motility usually begins immediately after germina- 

 tion. Gottheil in examining the germination of a number of soil 

 bacteria found that the time of the inception of movement varied for 

 different species from 7-12 hours after "sowing" the spores. When 

 once started, in the majority of cases, movement continues as long as 



