SUBFAMILIES AND GENERA OF COCCACEyE 79 



between the lemon yellow of such types as M. luteus and 

 the orange of M. aurantiacus (Schrb'ter-Cohn). On the 

 other hand, medical bacteriologists have recognized white, 

 yellow, and orange forms among the staphylococci of the 

 skin ; but it has been maintained by competent investigators 

 that these differences were scarcely of even varietal rank. 



A study of the distribution of our five hundred cultures 

 in regard to pigment production showed, however, that 

 the four common types of chromogenesis represent real 

 centers of variation. Chromogenesis was studied, as 

 described in the preceding chapter, by growing each 

 culture for two weeks on agar at 20 degrees, spreading a 

 portion of the growth on white paper, drying it and then 

 matching against the color chart reproduced as the frontis- 

 piece of this volume. This chart includes nine different 

 hues, one white, three yellows, two oranges, and three reds, 

 with nine different chromas, or depths of color, under 

 each hue. The chroma number indicates in each case 

 the number of washes of the pure color used to produce 

 the effect reproduced in the figure. In the figure each 

 vertical column represents one hue, and the numerical 

 values at the side correspond to the chromas. The 

 transparent sheet, which may be superimposed over the 

 frontispiece, shows the distribution of the five hundred 

 cocci studied according to their pigment-producing power. 

 The general habitat of each organism is indicated by 

 the symbol used, a cross standing for a strain isolated 

 from the body, and a circle for one of saprophytic origin. 



