304 BACTERIOLOGY. 



axis. In some preparations involution-forms, consisting 

 of rods a little clubbed at one extremity or slightly 

 bulging at different points, may be detected. Branch- 

 ing forms of this organism have been described. It 

 varies in length — sometimes being seen in very short 

 segments, again much longer, though never as long as 

 threads. On an average, its length is seen to vary 

 from 2 to 5 //. 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. 61, 

 page 279.) 



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 com- 

 position that renders it more or less proof against the 

 simpler dyes. It is therefore necessary that more 

 energetic and penetrating 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 increased. Two of these substances are 



