302 BAGTEBIOLOGY. 



and this in turn is sealed up and retained at the same 

 temperature. Once having obtained the organism in 

 pure culture, its subsequent cultivation may be con- 

 ducted upon the glycerin-agar-agar mixture — ordinary 

 neutral nutrient agar-agar to which 6 or 7 per cent, of 

 glycerin has been added. This is a very favorable 

 medium for the growth of this organism after it has 

 accommodated itself to its saprophytic mode of exist- 

 ence, though blood-serum is perhaps the best medium 

 to be employed in obtaining the first generation of the 

 organism from the tubercular tissues. 



The organism may be cultivated also on neutral milk 

 to which 1 per cent, of agar-agar has been added, also 

 upon the surface of potato, and likewise in meat-infu- 

 sion bouillon containing 6 or 7 per cent, of glycerin. 



Cultures of the tubercle bacillus are characteristic in 

 appearance — after once having seen them there is but 

 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. 

 lumps of mealy looking granules. They are never 

 moist, and frequently have the appearance of coarse 

 meal which has been 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 making up the growth adhere so 

 tenaciously together that it is with the greatest diffi- 

 culty that they can be completely separated. In even 

 the oldest and dryest cultures pulverization is impos- 



