THE RELATION BETWEEN LIGHT AND PIGMENT-FOKMATION. 559 



and tlie conditions of the experiment were such as to flood 

 the animals witli monochromatic light on all sides. The 

 weed chiefly used for food was the natural food-plant, 

 Ceramium; a little fine green weed was used in one of the 

 i-ed light experiments. The vessel was surrounded on three 

 sides by the fluid colour screen and rested on a faintly re- 

 flecting surface, so there was no strong background effect. 

 The light employed was direct and diffused sunlight, and the 

 effects of heat and of ultra-violet rays were largely obviated 

 by the conditions of the experiment. 



The results of the experiment are given in 'J'able III, pp. 

 577, 578, and show that whilst the Hippoly te, in white light, 

 developed into brown forms containing both red and yellow 

 pigment in about equal proportions, those in red light passed 

 through a brown stage, but ultimately (three weeks) became 

 green, some remaining, however, reddish-yellow in 1909, 

 whilst the survivors in green light became bright carmine. 

 In other words the ultimate colour in this experiment is the 

 complement of that of the incident light. 



The details of the end-result show clearly that the green 

 Hippolyte produced in red light and the crimson Hippo- 

 lyte produced in green light are peculiar and distinctive. 

 The former possess yellow pigment in a maximally expanded 

 state, and such little red as they possess is of a vermilion tint. 

 ^loreover, the yellow is of a distinctly greenish tinge and is 

 accompanied by either very little diffuse blue or none. Thus 

 the green colour in these experimental specimens under red 

 light is largely due to an increase in the amount and quality 

 ■of the yellow pigtuent accompanied by contraction of the 

 formerly dominant red pigment (PI. 23, figs. 6 and 9). 



The crimson Hippolyte produced in green light is no less 

 distinctive (PI. 23, fig. 5). In contrast to the usual type of 

 red forms, the yellow pigment has completely disappeared 

 and the chromatophores are entirely filled with a deep carmine 

 pigment suffused with a bluish tinge. The general deep 

 carmine colour was new to me. ^Moreover, the chromatophores 

 on the surface of the eye-stalks were abnormally developed. 



