§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 105 



cases. The difference may therefore be attributed to the secretion 

 of a large amount of the concentrating hormone, W, by the pars 

 tuberaUs in response to the white background (Hogben and 

 Slome, 1936). 



The toads always remain pale on a black background after 

 removal of the source of B in the posterior lobe of the hypophysis. 

 If the whole adenohypophysis, including the pars tuberalis, is 

 removed, the toads become permanently dark. That this is due to 

 the loss of the source of W is claimed from finding that, after 

 slightly incomplete hypophysectomy of Xenopus, the pars tuberalis 

 may regenerate, in which case the lost response to a white back- 

 ground is regained (Waggener, 1930). 



As in Ligia^ it is claimed that the adaptive colour changes are 

 controlled by the secretion of the two hormones in different pro- 

 portions in response to illumination of different parts of the retina 

 of the eye by direct and reflected light (Fig. 3-23). The eyes of 

 Xenopus are on the dorsal surface of the head and direct light 

 stimulates the "floor" of the retina; on a black background no 

 other light reaches the eye and the secretion of the dispersing 

 HORMONE, B, is induced. Scattered light from a white background 

 stimulates the ''peripheral" part of the retina and induces secretion 

 of the concentrating hormone, W, whatever position the toad 

 adopts (Hogben and Slome, 1936). In the eel, Angiiilla, where the 

 eyes are lateral, the ventral and dorsal parts of the retina are as- 

 sumed to play the same role as floor and peripheral parts in the toad. 



These results seem to be conclusive; but they lack control 

 injections of saline or of some other non-active substance, and 

 they have not been confirmed by more recent work using better 

 techniques. It is possible, therefore, that there is no real difference 

 between Xenopus and the frog, Ranapipiens, in which there appears 

 only to be the one melanophore-dispersing hormone, B (Parker 

 and Scatterty, 1937), concentration of melanin resulting merely 

 from absence of B. 



Elasmobranchii. The background responses of the melano- 

 phores of the rough dogfish, Scyliorhinus, and other elasmo- 

 branchs are similar to those of the Amphibia, but they seem to be 

 even slower (Waring, 1938). There is no doubt that the melano- 

 phores are dispersed by B, since this can be extracted from their 



