382 A MANUAL OF BACTERIOLOGY 



media from 20 to 42 C. In gelatin plates the colonies 

 are visible in twenty-four to forty-eight hours. The deep 

 colonies are spherical, granular, and of a pale brownish 

 colour, darker at the centre than at the periphery. The 

 superficial colonies are at first punctate, round and almost 

 transparent, but subsequently spread on the surface and 

 may attain a diameter of 3 mm., their margins become 



FIG. 41. Colonies of the colon bacillus, superficial and deep. 



irregular, the surface is smooth, they are finely granular, 

 opalescent in appearance, and are thicker at the centre 

 than at the periphery (Fig. 41). On a gelatin streak a 

 copious white, shining, smooth growth develops, the mar- 

 gins of which are irregular and crenated (Plate XIII. c), 

 and in old cultures the medium becomes opalescent. In 

 a gelatin stab-culture a white growth develops along the 

 line of inoculation with one or more gas-bubbles. The 

 gelatin is not liquefied. On agar and on blood-serum a 

 thick, moist, shining, greyish layer forms. There is 

 abundant formation of gas in a stab-culture in glucose-agar 



