THE BACILLUS COLT 459 



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., the margins become 

 irregular, the surface is smooth, they are finely granular 

 opalescent in appearance, and are thicker at the centre 

 than at the periphery (Fig. 42). On a gelatin streak a 

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



FIG. 43. Colon bacillus. Gelatin shake culture showing gas production. 



gins of which are irregular and crenated (Plate XIV, b), 

 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 

 and in gelatin shake cultures (Fig. 43), provided the latter 

 medium be made with meat, for " lemco " gelatin 

 generally fails to give gas. On acid potato it forms a 



