VARIATION 



415 



luxuriant, raised, dry growth, yellow, pink, or brick- red in colour, with a coarsely 

 granular surface, resembling dry bread-crumbs (Figs. 74 and 75). This growth is 

 unlike that of any of the tubercle bacilli. A few strains, however, are non-pig- 

 mented. In glycerine broth some strains give a turbidity, but most of them do 

 not ; there is generally a surface growth, which may be thick and wrinkled like the 

 human tubercle bacillus, thick and coarsely granular, or more thin and delicate ; 

 a deposit of granular material is usual. Growth occurs in gelatin, but no liquefac- 

 tion is produced. The optimum temperature of growth varies, but is generally 

 in the neighbourhood of 31° C. All strains grow freely at room temperature ; several 

 grow at 45° C, while a few multiply even at 55° C. (see Schwabacher 19336). 



Fig. 73. — Myco. tubercu- 

 losis. 



Avian type. Glycerine 

 agar, 3 weeks, 37° C, 

 showing abundant growth 

 of butter-cream type. 



Fig. 74. — Myco. stercoris 

 (Mist bacillus). 



Glycerine agar, 3 weeks, 

 37° C., showing abundant 

 heaped-up growth. 



Fig. 75. — Myco. phlei I. 



Glycerine agar, 3 weeks, 

 37° C., showing abundant 

 growth with wrinkled 

 surface. 



Variation. — The phenomenon of variation in acid-fast bacilli has occupied the 

 attention of large numbers of workers in recent years. The field is a wide one, 

 but so far the results of exploring it have not been fruitful. Many contributions 

 to its study have been purely descriptive, and have been made without any apparent 

 realization of its biological significance. The terms used by different workers to 

 denote the variants they have observed have diflfered so widely that the nomen- 

 clature is in a state of the utmost confusion. Changes in virulence have been 

 postulated on inadequate grounds, and attempts to measure small differences 

 in virulence have often been confidently performed with numbers of animals quite 

 insufficient to provide a definite answer. 



One of the chief difficulties in work of this type lies in distinguishing 

 between fixed and environmental variants. Our own experience suggests that 

 a great many of the so-called variants are notbing more than adaptations to 



