20 



OUTLINES OF BACTERIOLOGY 



difi'erent stages in the development can be seen in Fig. 34. In this 

 case it is obvious that all the polar cilia of one individual arise together, 



Fig. 33. — Streptococcus pyogenes, showing cilia. 



develop at the same rate, and attain their full development at the same 

 time. It has been stated by several writers that the cilia arise, not 



Fig. 34. — To illustrate the development of cilia. Various stages of development 

 represented at a, 6, c, and d. 



from the interior, but rather direct from the membrane, the contention 

 being that the membrane is not altogether a lifeless layer, but partakes 

 somewhat of the nature of protoplasm itself. As, 

 however, the course of the cilia has been followed 

 through the membrane (Fig. 35), there can be no 

 doubt that they cannot arise from the membrane. 



Nature of Motility. In virtue of the possession 

 of cilia, bacteria are able to exhibit motion. Even 

 in solid nutrient media there is usually quite 

 enough water on the surface for the bacteria to 

 swim in, even although to our eyes it appears quite 

 dry. As seen in the various diagrams the cilia are 

 wavy in outline, and it is by their lashing that 

 the movement of the bacteria is caused. In the bacteria of the 

 coccus kind, movement takes the form of rotation in any direc- 



FiG. 35.— Spirillum 

 giganteum, showing 

 origin of cilia from 

 interior of cell. 



