8 TEXT-BOOK OF BACTERIOLOGY. 



If a needle be dipped into the covering and slowly drawn out 

 again, an elongating thread follows it which can be drawn out to 

 about 30 cm., and which consists of nothing but bacteria firmly ad- 

 hering together. 



The disposition of the membrane to form a jelly, and so to unite 

 in groups and form such coverings, is very different in the different 

 species of bacteria; in general it is particularly strong with the 

 mobile rod bacilli. 



It is still doubtful whether the membrane is the place of deposit 

 for the pigment in the micro-organisms which form coloring mat- 

 ter, and it is not yet definitely determined whether the formation 

 of the coloring matter takes place in the interior of the cells or 

 first in the substratum. A number of close observations would 

 seem to favor the latter possibility. In the Micrococcus prodigiosus, 

 for instance, the coloring matter lies separated in grains outside 

 the bacteria, and other colored products of change of matter, such 

 as a frequently-occurring greenish coloring matter, remain in solu- 

 tion outside the cells, and communicate themselves by diffusion to 

 the neighboring objects. 



A large number of rod and screw-shaped bacteria possess the 

 faculty of spontaneous movement that is, they have the power to 

 leave a place and make an independent change of location. 



For a long time we have been acquainted with the special organs 

 by which the movement is effected, and the existence of cilia, or 

 whipping threads (fiagella), was clearly shown and proved beyond 

 all doubt by R. Koch, with unstained bacteria, years ago. A par- 

 ticularly clever and careful observation was requisite to see these 

 things. The flagella are very delicate formations, with about the 

 same refractive power as the usual media employed as food-solu- 

 tion, etc. As they, moreover, continue throughout their activity 

 in very brisk movement, there is no possibility of seeing the flagella 

 in bacteria as they usually come under examination that is to say, 

 in a state of motion. All assertions to the contrary are based on 

 self-deception. 



Koch succeeded best in rendering the flagella visible by the fol- 

 lowing method : 



He took a drop of some putrid vegetable infusion algae, aqua- 

 tic plants, or dead leaves and placed it on a cover-glass. Before 

 it was quite dry he laid the cover-glass on the slide and examined 

 it with the highest magnifying power. Then he saw here and there, 

 wherever a trace of dampness still remained, a stranded bacillus, 

 or spirillum, and at the end of it the flagella, which, having no room 

 for action, lay exposed, and could even be shown in a photograph. 



