Chromatophores and Color Change 703 



do not permit one to decide between two possibilities: (1) all portions have 

 a black-dispersing and a white-concentrating action, with the brain and thor- 

 acic cord having in addition an antagonistic white-dispersing agent; and (2) 

 all portions have a white-dispersing and a black-dispersing action, but the 

 connectives alone contain a white-concentrating principle. 



Numerous reciprocal-injection experiments among Crago, Uca, and Pal- 

 aemonetes, in which the comparative distributions of Crago-darkening, 

 Crflgo-body-lightening, Uca black-dispersing, and Uca white-concentrating 

 activities within the various nervous systems are compared, show rather 

 clearly that CDH and LIDH cannot be the same, nor like CBLH or UWCH. 

 However, there is still a possibility that CBLH and UWCH are identical 

 since there is a qualitative parallel in their distributions within the three 

 nervous systems studied. Certain quantitative differences of activity, how- 

 ever, cast doubt on this identity. 



In summary, decapod crustacean nervous systems possess at least three 

 or four chromatophorotropins. Their roles in the total chromatic responses 

 of the whole organism have not been worked out for any one species. It 

 seems likely, however, that the nervous-system chromatophorotropins, togeth- 

 er with those from the sinus glands, will be shown to go far toward account- 

 ing for the intricate control of the crustacean pigmentary systems. 



A study of the time relations of melanophore changes in the isopod, Ligia 

 oceanica, has led Smith^^'^ to postulate a dual endocrine control of these 

 pigment cells, operation of both a darkening B-substance and a lightening 

 W-substance. Two antagonistic substances were postulated, since if only 

 a W-substance were present then the time of white to black change, wTb, 

 should be greater than white to darkness change, wTd, or darkness to black 

 change dTb, which was not found to be true. On the other hand if only a 

 W-substance were present, then bTw should be greater than bTd or dTw, 

 which is not the case. The data seemed reasonably explained in terms of 

 two factors with the B-substance more slowly eliminated from the blood 

 than the W-substance. This would account for the long bTd and the super- 

 normal phase of the wTd (Fig. 268). On the basis of experiments in which 

 different portions of the eye were opaqued, or differentially stimulated by 

 careful adjustment of the background, support was given for the view that 

 stimulation of dorsal elements of the retina results in production of B-sub- 

 stance, and stimulation of the lateroventral portions in production of W- 

 substance. 



Vertebrates. All the vertebrates possessing chromatophores show, in gen- 

 eral, a fundamental similarity in their functional organization of this sys- 

 tem. It seems profitable to develop the evolutionary trends separately within 

 each of the major divisions of the vertebrates in which functional chroma- 

 tophores are found. 



Amphibians. The amphibians may show in their early development a 

 period in which only primary color responses occur, the animals darkening 

 in light and becoming pale in darkness. This was originally described for 

 very young Axolotl by Babak" and has since been observed in very young 

 Rana pipiens''^ and Amhlystoma.^^^ These changes do not involve the eyes. 

 It has been suggested that this is a period during which the eyes are still 

 non-functional. ^^^ Other amphibians appear to show the secondary types of 



