CULTIVATION OF TUBERCLE BACILLUS 271 



medium with tubercular material free from other organisms, 

 there appear in from ten to fourteen days minute points of growth 

 of dull whitish colour, rather irregular, and slightly raised above 

 the surface (it is advisable to plant on the medium an actual 

 piece of the tubercular tissue and to fix it in a break of the 

 surface of the serum). Koch compared the appearance of these 

 to that of small dry scales. In such cultures the growths 

 usually reach only a 

 comparatively small size 

 and remain separate, 

 becoming confluent only 

 when many occur close to- 

 gether. In sub-cultures, 

 however, growth is more 

 luxuriant and may come 

 to form a dull wrinkled 

 film of whitish colour, 

 which may cover the 

 greater part of the sur- 

 face of the serum and at 

 the bottom of the tube 

 may grow over the surface 

 of the condensation water 

 on to the glass (Fig. 75, 

 A). The growth is always 

 of a dull appearance, and 

 has a considerable degree 

 of consistence, so that it 

 is difficult to dissociate a 

 portion thoroughly in a 

 drop of water. In older 

 cultures the growth may 

 "Scguire a slightly brown- 

 ish or buff colour. When 

 the small colonies are ex- 

 amined under a low power 

 of the microscope, they 

 are seen to be extending at the periphery in the form of wavy or 

 sinuous streaks which radiate outward, and which have been 

 compared to the flourishes of a pen. The central part shows 

 similar markings closely interwoven. These streaks are com- 

 posed of masses of the bacilli arranged in a more or less parallel 

 manner. 



On Dorset's egg medium and especially on glycerin egg 



a v, c 



Fig. 75. — Cultures of tubercle bacilli on 

 glycerin agar. 



A and B. Mammalian tubercle bacilli of human 

 type ; A is an old culture, B one of a few 

 weeks' growth. 



C. Avian tubercle bacilli. The growth is whiter 

 and smoother on the surface than the others. 



