Generic Characters in the Coccaceae i6i 



weight in German systems of classification. Certain special char- 

 acteristics do, indeed, appear in old gelatin colonies of the cocci 

 after several weeks of incubation. Colonies may remain almost 

 spherical; or they may expand in fiat, disclike growths with terraced 

 edges. Sometimes a distinct boss appears at the center, surrounded 

 by a flatter area. The edges may be entire, or more or less deeply 

 scalloped, and the edges of the scallops may be produced inward in 

 folds. Concentric rings sometimes appear in the interior of the colony, 

 or zones of partially liquefied gelatin around its periphery. Some 

 of these characters vary without any apparent reason, as different 

 colonies on a plate show different characteristics; this is perhaps 

 due to differences in the position of the original cell relative to the 

 gelatin surface. 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 atmosphere 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. Those ditTerences which 

 are really characteristic of the organisms themselves appear to be 

 related to two fundamental powers : the general vigor of growth and 

 the liquefying power. It may be possible that other differences exist 

 in old gelatin colonies which are really characteristic, but in the 

 present state of knowledge it seemed best to omit the gelatin 

 plate in favor of more definite tests. Liquefying power and general 

 vigor of growth are observed in the gelatin stab and the agar streak 

 respectively. 



Gelatin tubes. — All our cultures have been studied in the gelatin 

 tube, but only the single character of the amount of liquefaction 

 has been systematically recorded. The distinction between difTerent 

 non-liquefying colonies lies in the amount of surface growth and the 

 color, both of which characters are more easily studied on the agar 

 streak. The character of the surface growth, Hke that of the gelatin 

 plate colony, does not appear in this group to offer any character 

 of diagnostic value, and all the cocci grow fairly well in the stab. 



