16 MORPHOLOGY 



peritrichous bacteria then appear to be surrounded by a mass of bubbles 

 (Fig. 8, e). 



The speed of movement of the bacteria is subject to great variations, 

 the cilia being often paralyzed by noxious physiological conditions. Lack of 

 oxygen, insufficient nutriment, and the accumulation of acid in old cultures 

 are all factors which bring about this paralysis, and it is only necessary to 

 remove them (by neutralization of the acid, by removing the cover-glass, by 

 the addition of sugar, asparagin, &c.) to at once restore movement to the 

 cilia. That bacteria are motionless is not therefore a proof that they belong 

 to a non-motile species, and in some cases much experience is necessary for 

 decision. 



The linear progression of bacteria through a liquid is always accompanied 

 by a rotation around the longer axis of the cell, as is the case with the 

 swarm-spores of the lower thallophytes. In the flagellate infusoria the end 

 of the cell bearing the flagellum is always in advance in swimming, and 

 probably the lophotrichous and monotrichous bacteria move in the same 

 way, a reversal of the path necessitating a revolution of 1 8o° around the 

 transverse axis of the cell. The peritrichous bacteria have a similar mode 

 of progression to that of the members of the other groups, but in addition 

 they often exhibit curious tumbling movements, the cell hurrying across the 

 field of view and turning somersaults the whole way. 



In Beggiatoa, one of the filamentous bacteria, remarkable slow oscilla- 

 tory movements of the threads are observed, isolated fragments of which are 

 also able to glide to and fro as diatoms do. Both these types of movement 

 occur too among the cyanophyceae, giving the name to one group of forms, 

 the Oscillariae. The cause of the phenomena is quite incomprehensible, 

 no organs of locomotion having been discovered, and the cell-wall being 

 apparently closed all round, so that the protoplasm cannot be perceptibly 

 extruded anywhere. It is, however, highly improbable that the motion is 

 independent of the protoplasm. More delicate methods of research will 

 perhaps enable us to clear up the mystery. 



The term flexile has been applied to filaments which, although per se 

 rigid and stiff, are not straight, but twisted and thrown into curves. It is 

 supposed that such flexile filaments have somewhat less rigid cell-walls, 

 which yield to the strain exerted on them by the cell contents. These 

 flexile threads occur in all trichobacteria, but in the haplobacteria the 

 membrane is always rigid. Kinks and twists in filaments seem to arise 

 frequently by the mechanical displacement of neighbouring cells, The 

 subject of flexility needs further investigation. 



Reproduction of Bacteria by Fission (11). 



Given suitable conditions of nourishment, bacterial cells, like those of 

 all other organisms, divide into two when they reach a certain size. In 



