300 



AMPHIBIA 



xii. 5 



the action of the secretion of the pituitary gland (Fig. 173). Move- 

 ments in the other chromatophores can also affect the colour, yellow 

 being produced by disarrangement of the guanophores and so on. 

 Other colours may contribute to the patterns, blue (though rarely) 

 by the absence of the lipophores, red by pigment in the lipophores. 

 Changes in the melanophores may be of two sorts, primary or 

 direct and secondary or visual. The primary response depends on the 



Fig. 173. Stages of dispersal of pigment in the melanophores in the web of 



the frog Xenopus, as used by Hogben to assess the melanophore index. 



(After Hogben and Slome, Proc. Roy. Soc. B. 108.) 



direct effect of light on the skin, causing expansion. The secondary 

 effect consists in contraction of the pigment if the animal is illumin- 

 ated on a light-scattering surface (light background) but expansion 

 (and hence darkening of the animal) when it is illuminated from above 

 on a light-absorbing (dark) background. There are, therefore, distinct 

 responses from different parts of the retina. Illumination of the dorsal 

 part produces contraction, of the ventral part expansion of the 

 melanophores. 



Hogben and his co-workers have shown that the control of the 

 colour change of amphibians is mediated by variation in the secretion 

 of the pituitary gland ; there is no direct nervous control of the melano- 

 phores. There is still some doubt whether the pituitary produces its 

 effects by means of one hormone or two. The most fully known in- 

 fluence is that of the posterior lobe, producing a B substance, also 

 known as intermedin, which makes the melanophores expand. Ex- 



