THE TUBERCLE BACILLUS. 217 



been allowed to solidify on cover-glasses, and which 

 has heen carefully kept in the moist chamber, is 

 inoculated with a small portion of one of these 

 colonies, the growth of the tubercle bacilli may be 

 observed from time to time with the microscope. 

 Peculiarly twisted chains of cells, like notes of inter- 

 rogation, are formed, which gradually become thicker 

 in the middle, while their pointed ends become ex- 

 tended along the surface of the nutrient medium. 

 It must, however, be mentioned that, even with the 

 greatest care, these cover-glass cultivations do not 

 always succeed. 



The bacilli grow in a similar manner on agar-agar, 

 but the cultures are more diflBcult to prepare. If a 

 small quantity (from 6 to 8 per cent.) of glycerine be 

 added to the agar-agar, the growth is much more 

 luxurious, but the glycerine interferes with the char- 

 acteristic appearance of the colonies. The more 

 glycerine which is added, the less dry are the scales, 

 the more do they flow into one another, and the more 

 extended are the colonies. 



These cultures of tubercle bacilli must be most care- 

 fully protected from the action of daylight. Direct 

 sunlight often kills the colonies outright in a few 

 minutes; even diffused light has the same effect in 

 a few days or weeks, according to its intensity. 

 Otherwise the cultures are very resistant, and only 

 require to be re-inoculated every two or three months. 



