Chromatophores and Color Change 



705 



melanin-concentrating action, called the W-substance. All the data obtained 

 appeared readily interpretable in terms of an excitation of secretion of the 

 W-substance by stimulation of peripheral retinal elements and excitation of 

 secretion of the B-substance by stimulation of basal retinal elements (Fig- 

 270). Furthermore, the data require the additional assumption that the W- 

 substance is added to the blood more slowly than the B-substance and also 

 disappears from the blood more slowly. The B-substance and the W-sub- 

 stance are antagonistic. Thus, according to this hypothesis, in an illuminated 

 environment B-substance is always secreted, but its secretion is reduced 

 in low illumination and in darkness. The responses to black and white illum- 

 inated backgrounds involve changes in the amount of the W-substance and 

 consequently changes in the B/W ratio. The very slow responses observed 

 in the change from an illuminated white background to darkness and the 



Brain 



Pars 



intermedia 

 ' (pituitary) 



0?Pars 

 tu 



tuberalis 



Blood vessel 

 Fig. 270. Schematic representation of two hypotheses of hormonal control of melano- 

 phores in Xenopus, one involving a single hormone, and the pther, two. From Hogben 

 and Slome.'"" 



reverse appears explainable in terms of concomitant decreases or increases, 

 respectively, of the two antagonistic substances. In the change from darkness 

 to an illuminated black background the melanophores pass through a super- 

 normal phase (more dispersed than typical for the background), a fact ex- 

 plainable in terms of a more rapid secretion of B-substance than of W-sub- 

 stance. As the latter increases to its full quantity, the melanin reaches its 

 slightly less dispersed final state. Similarly, in passing from an illuminated 

 black background to darkness the melanophore passes through a transitory 

 stage of greater concentration than the ultimate state, apparently due to a 

 more rapid reduction in B-substance than in W-substance. 



The only direct e\'idence for the existence of the W-substance in Xenofus 

 has come from obser\'ations on the responses of the animals to environmental 

 stimuli after various types of operative procedures.^*^ When the anterior lobe 

 of the hypophysis is removed the animal responds quite as it does normally. 



