BACTERIUM TUBERCULOSIS. 351 



Cultures of the tubercle bacillus are characteristic 

 in appearance once having seen them there is little 

 probability of subsequent mistake. They appear as 

 dry masses, which may develop upon the surface of 

 the medium either as flat scales or as coarse granular 

 masses. They are never moist, and frequently have the 

 appearance of dry meal spread upon the surface of the 

 medium. In the lower part of the tube in which they 

 are growing i. e., that part occupied by a few drops of 

 fluid which has in part been squeezed from the medium 

 during the process of solidification, and is in part water 

 of condensation the colonies may be seen to float as a 

 thin pellicle upon the surface of the fluid. 



The individuals composing the growth adhere so 

 tenaciously together that it is with the greatest diffi- 

 culty they can be separated. In even the oldest and 

 dryest cultures pulverization is impossible. The masses 

 can only 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 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 patho- 

 genic properties. 



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

 color as the medium that, unless they are growing as 

 characteristic mealy-looking masses, considerably ele- 

 vated above the surface, their presence is less conspicu- 

 ous than when on other media. 



