COMPARATIVE STUDY OF THE COCCI 69 



attribute the first, at least, to true variation rather than 

 to contamination. 



In spite of all these facts, it is clear that, as the cocci 

 normally occur in nature, chromogenesis is one of their 

 most distinct and significant differences. In any series 

 of plates sown with washings from the outer skin, three 

 well-marked types — yerlow, orange, and white — are 

 fairly certain to occur; and a fourth, red type is common 

 in air plates. Variations due to past and present environ- 

 ment are, of course, easily excluded by the maintenance 

 of constant conditions. Our stock cultures were in all 

 cases kept on agar at 20 degrees, and cultures for 

 chromogenesis were grown on that medium, and at that 

 temperature, for two weeks. In order to avoid the 

 apparent differences due to vigor of growth or to evapora- 

 tion a fixed portion of the growth was removed on a 

 loop needle and spread evenly on white drawing-paper 

 with a rough surface. After drying at the room tem- 

 perature, the color was compared with an arbitrary 

 standard scheme. 



The color chart used for matching these colors was 

 devised after a very careful study of the colors actually 

 found among the Coccaceae, and includes nine hues, rang- 

 ing from white through lemon-yellow, light cadmium, 

 medium cadmium, lemon-yellow and cadmium orange, 

 cadmium orange, red and cadmium orange, to two 

 other shades of red. We have used, under each hue, 

 nine different chromas, obtained by successive washes 



