TEXT-BOOK OF BACTERIOLOGY. 37 



The form of the bacteria, it is true, can be recognized only to 

 a certain extent, and all the peculiarities of form cannot be seen 

 without the application of special means. 



But the outlines are seen sharply and clearly, we observe the 

 turbid, equable contents of the separate cells, now and then also 

 a slight granulation, or, in the interior of the germs, those bright, 

 gleaming, egg-shaped bodies, or spores of the bacteria, may be de- 

 tected. 



The ability of voluntary movement is seen very beautifully and 

 clearly with the hollow slide in several of the bacteria; in fact, this 

 kind of examination is the only one that can yield unobjectionable 

 and really reliable information regarding this important function 

 of the micro-organisms. 



It is true that slight changes of place made by micro-organisms 

 which have not the power of self-movement may be noticed. But 

 on closer observation, it will be seen that this is not true locomo- 

 tion, but only a motion at and about a point, the so-called Brown's 

 or molecular motion. The globular bacteria which, as a rule, are 

 non-motile, in particular, almost always show this peculiar danc- 

 ing up-and-down movement. But it is very different from the de- 

 cided, almost self-conscious manner in which many of the rod and 

 screw-shaped bacteria move. Differences of gait may even be dis- 

 tinguished. The typhus bacilli taken from a potato culture, in ser- 

 pent-like windings glide nimbly across the field of vision ; the hay- 

 bacilli waddle along, bending from side to side as they go; while 

 the Bacillus megaterium crawls along with his peculiar amoeboid 

 movement. Quite different is the scene when examining the blue- 

 milk bacilli, the green-pus bacilli, or even the cholera bacilli. There 

 all appears in a confused jumble, like "a swarm of dancing gnats," 

 and the eye of the observer can scarcely distinguish the individ- 

 uals in the moving mass. 



The special motile organs of the bacteria, the flagella, it is 

 true, cannot be seei^r by this method of observation ; as already 

 noted, it requires other conditions to make them visible. Another 

 particularly valuable property of the hollowed slide consists in the 

 outside air being kept out, so that no considerable amount of evap 

 oration can take place from the surface of the liquid. Therefore 

 the hanging drops are indispensable for all examinations of long 

 duration; they can be kept for days, and even weeks, and that at 

 pretty high temperatures, without drying up; and (as will be de- 

 scribed later) this quality also enables us to utilize them in our 

 breeding processes. 



Beyond this point we cannot advance with unstained bacteria. 



