§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 103 



Amphibia. The background responses of Amphibia are similar 

 to those of the Crustacea in so far as the animals become pale on 

 an illuminated white background and dark by melanophore 

 dispersion on an illuminated black background. Early investiga- 

 tions of these colour changes were made on the African clawed 

 toad, Xenopus and attempted to use the different rates of change 

 of the chromatophores, following changes of background or 

 changes to and from darkness (Hogben and Slome, 1931), to 

 indicate the number of hormones that were necessary to control 

 the changes. The observations were good in that they did not 

 interfere with the integrity of the animals, but recorded their 

 natural physiological reactions. But the changes were so slow and 

 were interfered with by extraneous factors like diurnal rhythms 

 and unsuitable temperatures, so that the results were unsatis- 

 factory. Recourse to the pharmacological method of injecting 

 extracts into variously prepared specimens was therefore necessary 

 to obtain conclusive evidence for the presence of two hormones. 



The dispersing or melanophore-stimulating hormone, 

 MSH, (known as B to the earlier writers) can be easily established 

 by injection of extracts of the posterior lobe of the frog hypophysis, 

 although the early experiments made use of mammalian extracts. 

 The source is the pars intermedia (§ 2.123). 



It has not been possible to prepare active extracts of the antago- 

 nistic concentrating hormone, known as W and believed to be 

 secreted from, or controlled by, the pars tuberalis (§ 2.123). The 

 best evidence for its presence is obtained by injecting equivalent 

 extracts of active B substance into variously prepared test toads, 

 and comparing results on a white background (Hogben and Slome, 

 1936). 



Injecting a standard dose of B into the normal pale animal 

 produces a temporary increase in melanophore dispersion that 

 wears off in about 10 hr, when the toad's normal response again 

 takes charge (Fig. 3-22, curve B). 



If the whole hypophysis is removed, including both the known 

 source of B and the postulated source of W, a much greater effect 

 (Fig. 3-22, curve A) is produced for the given dose of B than in 

 the intact animal. The effect also persists longer, just as if the 

 natural supply of W were absent. 



