444 APPLICATION OF METHODS OF BACTERIOLOGY 



ciously together that it is with the greatest difficulty they 

 can be separated. In even the oldest and dryest cultures 

 pulverization is impossible. The masses can only be sepa- 

 rated and broken v up by grinding in a mortar with the 

 addition of some foreign substance, such as very fine, 

 sterilized sand, or ground glass, etc. 



The cultures are of a dirty-drab or brownish-gray color 

 when seen on serum or glycerin-agar-agar. 



On potato they grow in practically the same way, though 

 the development is much more limited. On this medium 

 they are of nearly the same color as the potato on which 

 they are growing. When cultivated for a time on potato 

 they are said to lose their pathogenic properties. 



On milk-agar-agar they, are of so nearly the same color 

 as the medium that, unless they are growing as character- 

 istic mealy-looking masses, considerably elevated above the 

 surface, their presence is less conspicuous than when on 

 other media. 



In bouillon they grow as a thin pellicle on the surface. 

 This may fall to the bottom of the fluid and continue to 

 develop, its place on the surface being taken by a second 

 pellicle. 



The, tubercle bacillus does not develop on gelatin because 

 of the low temperature at which this medium must be used. 



Microscopic Appearance of Bacterium Tuberculosis. Micro- 

 scopically the organism itself is a delicate rod, usually 

 somewhat beaded in its structure, though rarely it is seen 

 to be homogeneous. It is either quite straight, or somewhat 

 curved or bent on its long axis. In some preparations 

 involution-forms, consisting of rods a little clubbed at one 

 extremity or slightly bulging at different points, may be 

 detected. Branching forms of this organism have been 



