214 BACTERIOLOGY. 



oldest and dryest cultures pulverization is impossible. 

 The masses can ouly be separated and broken up by 

 grinding in a mortar with the addition of some foreign 

 substance, such as very fine, sterilized sand, dust, etc. 



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

 color when seen on serum or on glycerin-agar-agar. 



On potato they grow in practically the same way, 

 though the developnient is much more limited. They 

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

 they are growing. 



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

 color as the medium, that unless they are growing as the 

 mealy-looking masses considerably elevated above the 

 surface, their presence is less conspicuous than when on 

 the other media. 



In bouillon they appear as a thin pellicle on the sur- 

 face. This may fall to the bottom of the fluid and con- 

 tinue to develop, its place on the surface being taken 

 by a second pellicle. 



Under all conditions of artificial development the 

 cultures of this organism are always very dry and 

 brittle in appearance, though in truth the individuals 

 adhere tenaciously together by a very glutinous sub- 

 stance. 



The tubercle bacillus does not develop on gelatin, 

 because of the low temperature at which this medium 

 must be used. 



Microscopic Appearance of the Tubercle 

 Bacillus. — Microscopically 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, consist- 



