GENERAL MORPHOLOGICAL CHARACTERS. 159 



the mobile forms, is in some species impregnated with 

 colouring matter. On the outside a further gelatinous 

 sheath may be recognised which is developed to a very 

 variable degree in different species of bacteria. At 

 times it is very distinct, especially after the use of 

 staining materials, and surrounds the individual cells 

 as a broad zone, the outer contour of which corresponds 

 to the form of the cell ; at times the existence of the 

 sheath can only be suspected from the spaces left 

 between the closely packed cells. 



Some vegetative forms and species of bacteria are independent 

 always at rest ; thus the spherical cells, and all those 

 species which only occur in the form of micrococci, bacteria, 

 exhibit only a trembling movement with very slight 

 alteration of position which may be referred to unavoid- 

 able agitation and currents ; they are never seen to 

 travel over considerable distances. Other forms, bacteria, 

 bacilli, and spirochaete are sometimes at rest, sometimes 

 in active movement. This movement consists partly in 

 rotation around their long axis and partly in bending 

 and stretching; in many cases the action of flagella 

 seems to be the cause of the commencement and main- 

 tenance of the movement. These flagella can be demon- 

 strated with complete certainty in some 

 spirilla and bacilli ; but even in them 

 they can only be seen with the best 

 lenses, and after special treatment 

 of the preparation (drying and staining 



.., , i , -ii Fig. 35. -Bacilli and 



with gentian violet, or better with con- gp f ri iia with flagella. 

 centrated watery solution of hoBmatoxy- 

 lin). At times their presence is only betrayed by the 

 peculiar whirling movement of the fluid surrounding the 

 ends of the cells. The movement of the swarming bacteria 

 takes place forwards and backwards in the direction of 

 the long axis ; at times it is slow, oscillating and roll- 

 ing, at times active and dcarting, so that the field of the 

 microscope is traversed in a moment. Various alter- 

 ations of the external conditions, such as change of 

 temperature, exclusion of oxygen, &c., cause slowing 

 and cessation of the movement. 



