DESCRIPTION OF CULTURES 103 



the reader may be able to follow the reasoning which 

 led us from one series of experiments to the next, but 

 also because we believe that the same fundamental 

 laws will be found to apply to Johne's bacillus and to 

 the lepra and tubercle bacilli. We hope, too, that 

 the experiments that we have carried out may help to 

 throw some light on the vital chemical changes that 

 occur in the life-history of these bacilli, both when 

 outside and when inside the animal body. 



Description of Cultures.— In giving the experiments 

 on the cultivation of Johne's bacillus, we have fre- 

 quently described the cultures obtained, but in most 

 cases the description refers only to young primary 

 growths, so that a more complete description of 

 vigorous growing and older cultures is not out of 

 place. Details of the preparation of the media have 

 already been given. 



Egg Media containing the Essential Substance. — These 

 media, as we have seen, may be made with tubercle 

 or other acid-fast bacilli that contain the essential 

 substance, or with the essential substance extracted 

 from these bacilli ; they must contain about 4 per cent, 

 by volume of glycerine. On such media Johne's 

 bacillus in primary cultures grows as tiny, dull white 

 colonies, which are rarely visible to the naked eye in 

 less than four weeks. The colonies are irregularly 

 round, and either remain quite small and discrete, or 

 they may coalesce. If only a few are present, they 

 increase in size, and when older become more elevated, 

 especially in the centre, and turn dull yellowish-white 

 in colour. The edges of the colonies remain thin, 

 and from the margins numerous irregular elevations 

 gradually rise towards the centre, and end in a peak, 

 which is sometimes very markedly elevated. The 

 elevations are most marked on dry media and on those 



