DESCRIPl^ION OF CULTURES 105 



on two tubes of one such medium, which contains dog's 

 unheated ascitic fluid which had been passed through 

 a sterile Doulton white filter, are shown in Plate VI., 

 Figs. 2 and 3. 



Fluid Nutrient 7l/^^/<7.— Ordinary peptone-beef broth 

 containing glycerine and the essential substance, or a 

 bacillus containing the essential substance, does not 

 form a good medium for a freshly isolated strain of 

 Johne's bacillus. There is great difficulty in inducing 

 the bacillus to grow on the surface of the medium. 

 After three or four weeks, growth occurs at the bottom 

 of the flask as tiny yellowish-white grains, and these 

 gradually increase in size and number. It is usually 

 some months before any growth takes place on the 

 surface of the medium, and when it first appears it 

 occurs as a very delicate film. In subcultures the 

 growth becomes thicker and more vigorous, although 

 in this case it rarely covers the whole surface of the 

 medium. After subculturing several times, surface 

 growth may occur as very thick, knobby, and irregular 

 masses. We found this knobby condition particularly 

 well marked with a strain of the bacillus which, after 

 two years, had become acclimatized to grow on 

 glycerine-beef broth without the essential substance ; 

 such a growth is shown in Plate VIII. The growth 

 extends Irom these knobby masses by forming fresh 

 rings of thin growth, which in turn also become heaped 

 up and knobby, or in the same way entirely fresh areas 

 may form. Several of the thin patches may be seen 

 very faintly in the plate. As the bacillus becomes 

 better acclimatized to the glycerine-peptone-beef broth 

 medium, the growth ceases to be characteristically 

 knobby, and becomes more like that of the tubercle 

 bacilli. 



