56 RELATIONSHIPS OF THE COCCACE^E 



Some of these characters vary, without any apparent 

 reason, as different colonies on a plate show different 

 characteristics; this is perhaps due to the depth at 

 which the original cell lay below the surface of the 

 gelatin. Most of them are profoundly modified by 

 variations in the amount of moisture in the gelatin, and 

 in the atmosphere above. In a series of comparative 

 studies with different conditions of incubation we found 

 that highly characteristic colonies of granular structure, 

 with deeply lobed edges and indented surfaces, could be 

 produced by cultivation in an incubator whose atmos- 

 phere was kept dry by calcium chloride. Dunham 

 (1903) has pointed out the wide differences which may 

 be due to slight variations in the physical properties of 

 the gelatin used. In some groups of bacteria, the 

 Proteus group and the B. ramosus group for example, 

 colony forms are obviously characteristic. Among the 

 cocci, however, a slightly elevated disk appears to 

 be the prevailing form of colony under favorable con- 

 ditions of moisture and with the proper consistency of 

 gelatin. In our experience, all the constant differences 

 observed could be explained by variations in general 

 vigor of growth and in rate of gelatin liquefaction. 

 These two factors, liquefying power and general vigor 

 of growth, were observed on the gelatin stab and the agar 

 streak, respectively. 



Gelatin tubes. All our cultures have been studied in 

 the gelatin tube, and the rate of liquefaction has been 



