cei 



CULTURAL CHARACTERISTICS 



321 



cent, glycerin and are prepared in the customary way. " The potato is cut into 

 slices put in water and soaked for 24 hours in a 5 per cent, glycerin blue- 

 litmus solution, which is poured off before the first sterilization ; at the bottom 

 of each tube is some absorbent cotton-wool soaked with glycerin solution 

 which helps to keep the potato moist." 



5th Generation 

 over 3 months old. 



6th Generation 

 55 days old. 



FIG. 211. Cultures of tubercle bacilli of human origin on glycerin-gelatin, 

 (a) A bovine tubercle bacillus of intermediate virulence for calves and rabbits ; 

 (6) an human tubercle bacillus slightly virulent for rabbits (A. S. Griffith). 

 (See footnote p. 316.) 



[F. Griffith states that "it is possible to differentiate between the avian 

 and the mammalian types of tubercle bacilli from cultural characters alone, 

 but it is necessary in order to avoid error, that a sufficient variety of media 

 should be used. In primary cultures on serum or serum agar the transparent 

 colonies of avian tubercle bacilli are easily recognized. On glycerin-agar 



