Generic Characters in the Coccaceae 



195 



moderate liquefiers, and the orange chromogens exhibit the 

 peptonizing power to a high degree. 



To sum up, the cocci show four (or five) distinct groups according 

 to their pigment production, each group being marked by a number 

 of other correlated characters. The "white" forms rarely show 

 packets, generally stain by Gram, fail to reduce nitrates, grow well 

 at 37°, and usually fail to licjucfy gelatin. They include two sub- 

 groups — the feebly growing, strongly acid-producing forms, which 

 are really colorless, not white, and the white-pigment-producers, 

 which grow abundantly and produce only a slight amount of acid. The 

 "yellow" chromogens frequently show packets, are usually Gram- 

 negative, give a good to a very heavy surface growth, produce little or 

 no acid, occasionally form nitrites or ammonia in nitrate solution, 

 grow well at 20°, and show a moderate liquefaction of gelatin. The 

 "orange "-pigment-formers are very rarely packets, stain well by 

 Gram, form good surface growths, produce a moderate acidity in 

 sugar broth, occasionally reduce nitrates to nitrites or ammonia, grow 

 well, but with poor pigment production, at 37°, and generally produce 

 a considerable liquefaction of gelatin. The red-pigment-producers 

 are often packets, generally Gram-negative, grow abundantly, ferment 

 dextrose but not lactose, form nitrites, but not ammonia, in nitrate 

 solution, grow well at 20° or 37°, producing less pigment in the latter 

 case, and generally fail to liquefy gelatin. 



9. Gelatin Liquefaction. 



In the routine determination of gelatin liquefaction we have 

 used only one tube for each culture. Duplicate determinations 

 were made on 79 cultures, from which it appeared that the probable 

 error of a single observation is only ±0.12 cm.; so that our method 

 was sufficiently accurate. 



Of the cultures observed, 186 failed to liquefy gelatin, and the 

 distribution of the other 314 cultures according to the amount of 



TABLE 42. 

 Gelatin Liquefaction. 



Depth in cm 



Number of cultures 



over 3.S 

 3 



