TEXT-BOOK OF BACTERIOLOGY. 165 



liquefaction and appear as round depressions with gray-colored 

 contents and well-defined edge. If we examine the plate uiader the 

 microscope we see, in the former case, irregularly-formed greenish- 

 brown granular masses, in the latter case (or surface colonies) 

 grayish-yellow dense masses, finely and evenly granulated. The 

 edge is fringed with short fibres; attentive observation shows a 

 backward and forward motion, a lively swaying to and fro in the 

 colonies, even with a low magnifying power. 



When the development proceeds still further the gelatin be- 

 comes slightly red, which gradually deepens and at last becomes 

 brick-red. 



Similar appearances are seen in test-tube cultures. All along 

 the puncture rapid liquefaction occurs; also accumulation of dense, 

 flaky, grayish-white masses in the deeper layers and a delicately- 

 folded, strongly-reddened film on the surface. Obliquely-hardened 

 agar is covered over in twenty-four hours in the incubator, in a few 

 days at room temperature with a shining crust, which after a 

 time, as a rule, shows the brick-red color over its entire surface. 



The best medium for the pigment development is the potato. 

 Here a thick, greasy growth quickly appears and soon takes the 

 faint red color. 



Yet the pigment formation is a very capricious quality of the 

 indicus, and is still more dependent on exterior influences than in 

 the case of the prodigiosus. Even under ordinary circumstances 

 the coloring sometimes fails to appear, or it appears in full 

 strength only on one part of the culture, the rest remaining pale. 

 The edges of potato or agar growths in particular are almost 

 always white; in the incubator no pigment is developed, and even 

 the immediate progeny of deep-red bacteria often show no tendency 

 to color. 



Like most of the parasitic species, the indicus possesses the 

 quality of acting toxically on animals when administered in consid- 

 erable quantity. Guinea-pigs and rabbits are killed by the injec- 

 tion of 20 c.cm. of a fresh bouillon culture into the peritoneal cavity. 

 It is worthy of remark that death also takes place when the same 

 quantity is introduced into the circulation by injection into a vein, 

 but that in this case symptoms of intense inflammation of the in- 

 testinal mucous membrane are generally observed, which are some- 

 times even accompanied by the formation of ulcers. 



SARCINA. 



The bacteriological examination of the air has made us ac- 

 quainted with a number of different micro-organisms, few of which, 



