xii. 5 



COLOUR CHANGE IN AMPHIBIA 



301 



tracts of the pituitary of mammals (or other vertebrates) produce this 

 effect when injected into frogs, and after removal of the pituitary a 

 frog becomes pale in colour. There is also some evidence for secre- 

 tion bv the anterior lobe of a W substance that causes paling. After 

 removal of the whole pituitary the melanophores are found to be not 

 in the wholly contracted stage 1 of Hogben's melanophore index (Fig. 

 173) but in a state (stage 2 or 3) intermediate between this and full 

 expansion (stage 5). If, however, the posterior lobe alone is removed 

 the animal becomes completely pale (stage 1). This certainly suggests 



10 12 14 

 Hours 



Fig. 174. Effect of an extract containing the b (melanophore-expanding) substance on 



three groups of Xenopus. All were of the same weight and received the same dose. 



A, whole pituitary removed; D, intact animals; c, posterior lobe only removed. 



(From Waring, after Hogben and Slome.) 



the secretion of a W substance by the pars anterior. There are also 

 differences in the response to injection of B-containing extracts after 

 total and partial removal of the pituitary (Fig. 174). The position is 

 complicated by the fact that extracts of mammalian pineal or adrenal 

 medulla will cause contraction of amphibian melanophores, though it 

 is uncertain whether these effects have any physiological significance. 



In amphibia there is no direct control of the pigment cells by 

 nerve-fibres such as are present in bony fishes (p. 259). The colour 

 change is therefore rather slow. After removal of the pituitary the 

 melanophores still show slight changes correlated with change of in- 

 cident illumination, indicating a small degree of direct response as 

 independent effectors. Temperature and humidity also influence the 

 colour in many amphibia. In frogs contact with water accentuates the 

 black-background response and in darkness produces expansion. On 

 the other hand, drying induces contraction of the melanophores, even 

 upon a black background. 



The colour patterns adopted are usually cryptic or concealing in 

 their effect, but the colour also has an important influence on the 

 temperature and varies with it and with the humidity, as well as with 

 the incident illumination. The uniform brilliant green of tree-frogs 



