132 COMPARATIVE PHYSIOLOGY 



salamander were carried out by the writer (Hogben, 1924). 

 The outstanding results of these researches may be epitomised 

 as follows : — 



1. After removal of the whole pituitary in adult frogs, 

 as in Axolotls and Anuran tadpoles (Smith, Allen, and Atwell), 

 the individuals so treated remain permanently pale with the 

 melanophores in maximum contraction, although subjected 

 to conditions which inevitably induce darkening of the skin 

 in normal animals. 



2. Melanophore expansion follows pituitary injection in 

 hypophysectomised individuals ; but hypophysectomised indi- 

 viduals so treated regain their characteristic pallor, although 

 subject to conditions which inevitably evoke melanophore 

 expansion in the normal animal. 



3. A comparison of the minimal standardised dose of a 

 sample of pituitary extract requisite to induce melanophore 

 expansion in normal and hypophysectomised frogs, under 

 conditions in which the intensity of external factors tending 

 to promote pallor were varied, favours the view that melano- 

 phore contraction and expansion in the intact animal is 

 correlated with the amount of pituitary secretion in the 

 circulation. 



The regulation of colour-response by fluctuating pituitary 

 secretion is thus adequate to interpret all the accredited 

 phenomena in adult Amphibia, without invoking a direct 

 innervation of melanophores. We may justifiably conclude 

 that in Urodeles as in Anura pituitary secretion is controlled 

 by various (e.g. thermic) receptors in the skin, and is reflexly 

 inhibited by light acting on the retina. This fully explains 

 why in the salamander Diemyctilus (Rogers), within the 

 range of external conditions for which light is the significant 

 factor, section of the optic nerve was found to result in per- 

 manent melanophore expansion, although transection of the 

 cord was without eflPect on the rhythm of colour-response. 



It may here be noted that there is no conclusive evidence in 

 favour of the existence of a direct nervous control of the 

 melanophores in amphibia. Contraction of melanophores 

 is brought about in frogs by injection of adrenalin ; but 



