26 THE GROUSE DISEASE chap. 



about a week has covered nearly the whole free 

 surface of the gelatine in the test-tube. But even 

 at the maximum growth the dots constituting the 

 stab remain small (Figs. 6, 7), and are brownish in 

 transmitted light. 



After a few days' incubation of the stab culture there 

 are generally in the deeper parts one or two large 

 flat gas-bubbles to be noticed, fixed on to the growth 

 with one end, and placed, therefore, eccentrically to 

 the line of the growth. The gas -bubbles slant 

 generally in an upward direction. The development 

 of gas-bubbles in the deeper parts of the growth is 

 well and easily shown by making a " shake culture " 

 in gelatine — that is to say, by distributing (by shak- 

 ing) a limited number of the microbes in melted 

 nutrient gelatine contained in a test-tube and then 

 allowing this gelatine to set, by keeping it in cold 

 water for a short time before incubation. After 

 two days numerous colonies are visible as minute 

 dots distributed through all the layers of the gelatine, 

 and attached to each dot or colony in the deeper 

 parts is a small flat gas-bubble. Such a culture 

 looks extremely characteristic (Fig. 8). The gas- 

 bubbles increase in size, and, if the colonies are 

 numerous, i.e. if a considerable number of the 

 microbes had been originally introduced into the 

 gelatine, the lower half or two-thirds of the culture 



