TEXT-BOOK OF BACTERIOLOGY. 181 



On the glass plate the naked eye perceives, first of all, small 

 white dots in the mass, while at the surface thick bluish-white 

 knobs, which shine like porcelain, swell up. Under the microscope 

 both kinds appear as dark-brown solid discs, with a smooth, sharp 

 edge. Toward the margin the strong coloring fades a little, but 

 there is nowhere to be seen any delicate pattern- work in the color- 

 ing, nor even a distinct granulation of its contents. 



In the test-tube cultures the blue-milk bacilli show a decided 

 tendency to surface-growth. Thus the development generally fails 

 entirely in the lower part of the puncture; higher up a thin white 

 thread is formed, while the chief mass of bacterial growth spreads 

 over the surface of the gelatin as a dirty -white or light-gray cover- 

 ing. 



The gelatin is never liquefied, but it gradually takes a peculiar 

 discoloration which proceeds from the culture, and by no means 

 always shows itself in the same manner. The differences which 

 are observed depend on the reaction of the gelatin. The less alka- 

 line it is the more clearly is the blue color seen, and in slightly-acid 

 gelatin the formation of coloring matter reaches its maximum. 



Agar-agar also shows this discoloration, while the mass of bac- 

 teria itself is seen as a dirty-gray, moist, thickish covering in the 

 immediate neighborhood of the puncture. 



The growth on potatoes is very characteristic. The whole sur- 

 face of the slice is quickly covered by a thick, fatty mass, which 

 saturates the culture medium with pigment. Thus the whole 

 potato appears discolored up to its very edge, varying from 

 blackish-blue to yellowish-brown, according to its reaction. 



If we transfer a small portion of such a pure culture into ordi- 

 nary unboiled milk (skimmed milk being, as Heim has shown, the 

 best for the purpose) some peculiar changes take place. First of 

 all, a few blue spots appear on the surface ; these coalesce, the milk 

 becomes covered with a colored film, at the same time the casein 

 coagulates and the fluid becomes distinctly acid. It is otherwise 

 if sterilized milk be inoculated in the same way. Then the precipi- 

 tation of casein does not take place, the alkaline or amphoteric 

 reaction is not altered, and the discoloration is by no means so 

 striking as in the first case, and instead of being " sky-blue," the 

 milk appears slate-gray or dirty-violet. This shows clearly enough 

 that the blue-milk bacilli are only able to produce the pigment 

 without being concerned in any of the other changes which take 

 place in the milk, and which are all caused by other organisms, 

 two of which have already been noticed. 



The coloring matter is formed outside the rod-cells at the ex- 



