COMPARATIVE STUDY OF THE COCCI 55 



aggregates. Both turbidity and sediment vary markedly 

 with the age of the culture; what is first turbidity 

 later settles to form sediment. The amount of either 

 depends on the activity of growth. A constant differ- 

 ence often appears between cultures which early in the 

 course of development show considerable turbidity with 

 little or no sediment; and those which almost at once 

 develop a heavy sediment with colony-like masses of 

 growth clinging to the walls of the tube. This difference, 

 however, appears to be correlated with the growth- 

 form and general vigor of the coccus. Organisms of 

 the Streptococcus type with cells separating readily, 

 which show faint surface growth, produce chiefly tur- 

 bidity; while organisms likeSarcina, with large cell aggre- 

 gates and rich surface growths, show heavy sediment. 



Gelatin plates. Minute differences in the macroscopic 

 and microscopic appearance of colonies on gelatin are 

 given great weight in German systems of classification. 

 Certain special characteristics do, indeed, appear in old 

 gelatin colonies of the cocci after several weeks of incuba- 

 tion. Colonies may remain almost spherical, or they 

 may expand in flat, disk-like 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. 



