ENDOCRINE CO-ORDINATION 129 



on a balance of natural factors of which humidity, temperature, 

 oxygen supply, and illumination are the most significant. 

 Dryness in the terrestrial species promotes pallor (contraction 

 of the melanophores). Complete melanophore expansion can 

 only occur in natural surroundings in the presence of moisture. 

 Warmth tends to produce pallor. Most commonly bright 

 light has the same effect, at least after continued exposure. 



Colour-responses require periods of several hours or even 



days to reach their maximum intensity. The following 



citation is taken from Laurens' paper on Amblystoma larvae : — 



" I. Expansion of the melanophores of seeing larvae in 



the light one and a half to two hours. 



"2. Expansion of the melanophores of eyeless larvae in 

 the light two to three hours, and contraction of the melano- 

 phores of seeing larvae in darkness two to three hours. 



"3. Contraction of the melanophores of eyeless larvae in 

 darkness four to five hours. 



** 4. ' Secondary ' contraction of the melanophores of 

 seeing larvae in the Hght three to five days, and ' secondary ' 

 expansion of the melanophores of seeing larvae in darkness 

 five days or more." 



Laurens does not give the temperature conditions to which 

 such periods are subject. 



" Adaptive " response to background colour depends on 

 stimuli for which the retina is the receptor. The characteristic 

 darkening of the skin in the breeding season among Anura 

 is probably independent of the internal conditions incident 

 to reproduction, and results from exposure to optimum con- 

 ditions for melanophore expansion during the period of coupling 

 and ovulation. The normal pigmentary responses of the frog 

 are summarised in the table on next page. 



The synchronous character of the colour responses which 

 occur in Amphibia implies that a regulatory mechanism 

 controls their pigmentary reactions. To earlier workers 

 nothing seemed more natural than the assumption that this 

 mechanism must be the nervous system. Every effort was 

 directed to seek a solution of the problem along this line. 

 The result has been a bewildering conflict of evidence from 



K 



