CULTIVATION OF TUBERCLE BACILLUS 277 



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. 80, 

 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 

 acquire 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 mark- 

 ings closely interwoven. These streaks are composed of masses 

 of the bacilli arranged in a more or less parallel manner. 

 k On Dorset's egg medium and especially on glycerin egg 

 medium the organism grows well, producing an abundant 

 wrinkled layer which has usually a yellowish, buff, or pinkish 



FIG. 80. 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. 



