32 CELL STRUCTURES 



from place to place or even out of the field of the microscope 

 with greater or less speed. There is a marked difference in 

 the character of the motion of different kinds of bacteria. 

 Some rotate around the long axis when moving, others 

 vibrate from side to side. 



Among the higher thread bacteria there are some w^hich 

 show motility without possessing flagella. Just how they 

 move is little understood. 



Spores.— Under certain conditions some bacterial cells 

 undergo transformations w^hich result in the formation of 

 so-called spores. If the process is followed under the micro- 

 scope the changes observed are approximately these: A 

 very minute point appears in the protoplasm which seems to 



Fig. 27.— The smaller oval bodies in the middle of the field are free spores. 



act somewhat like the centrosome of higher cells as a " center 

 of attraction," so that the protoplasm graduall}^ collects 

 around it. The spot disappears or is enclosed in the col- 

 lected protoplasm. This has evidently become denser as it 

 is more highly refractive than before. In time all or nearly 

 all of the cytoplasm is collected. A new cell wall is devel- 

 oped around it which is thicker than the cell wall of the 

 bacterium. This thickened cell wall is called the "spore 

 capsule." Gradually the remnants of the former cell con- 

 tents and the old cell wall disappear or dissolve and the 

 spore becomes "free" (Fig. 27). 



If the spore is placed in favorable conditions the proto- 

 plasm absorbs water, swells, the capsule bursts at some 

 point, a cell wall is formed and the bacterium grows to 



