STAINING PECULIARITIES. 281 



an average, its length is seen to vary from 2 to 5 JJL. It 

 is commonly described as being in length about one- 

 fourth to one- half the diameter of a red blood-corpuscle. 

 It is very slender. (Fig. 55, page 256.) 



These rods usually present, as has been said, an ap- 

 pearance of alternate stained and colorless portions. It 

 is the latter portions which are believed to be the spores 

 of the organism, though, as yet no absolute proof of this 

 opinion has been established. 



At times these colorless portions are seen to bulge 

 slightly beyond the contour of the rod, and in this way 

 give to the rods the beaded appearance so commonly 

 ascribed to them. 



STAINING PECULIARITIES. A peculiarity of this 

 organism is its behavior toward staining reagents, and 

 by this means alone it may be easily recognized. The 

 tubercle bacillus does not stain by the ordinary methods. 

 It possesses some peculiarity in its composition that 

 renders it more or less proof against the simpler dyes. 

 It is therefore necessary that more energetic and pene- 

 trating reagents than the ordinary watery solutions 

 should be employed. Experience has taught us that 

 certain substances not only increase the solubility of the 

 aniline coloring substances, but by their presence the 

 penetration of the coloring agents is very much in- 

 creased. Two of these substances are aniline oil and 

 carbolic acid. They are employed in the solutions to 

 about the point of saturation. (For the exact propor- 

 tions see chapter on Staining Reagents.) 



Under the influence of heat these solutions are seen 

 to stain all bacteria very intensely the tubercle bacilli 

 as well as the ordinary forms. If we subject our prep- 

 aration, which may contain a mixture of tubercle bacilli 



