328 The Pigmentary Effector System. 



the Amphibia the stimuli received by the retina and transmitted to the 

 central nervous system are " indisputably" there " transformed and sent out 

 along motor nerves to the pigment cells." That the retina exerts a controlling 

 influence is admitted ; that the stimuli it receives are transmitted to the 

 central nervous system can hardly be questioned ; but that the control of the 

 pigment cells thereby is of a purely reflex character cannot be conceded till 

 the endocrine factor has been eliminated. The existence of an endocrine 

 substance capable of producing the reverse response to adrenalin,* raises the 

 possibility that the central nervous system may exercise its control over the 

 pigment cells partly at least through the secretory activity of the suprarenal 

 and pituitary glands. 



7. Summary. 



1. Extracts of the posterior lobe of the pituitary gland have a specific effect 

 on the melanophores of the frog causing them to undergo extreme expansion ; 

 an injection, equivalent to less than l/1000th of the ordinary clinical dose, is 

 adequate to produce a conspicuous darkening of the skin visible to the naked 

 eye. The effect of pituitary extract on the dermal melanophores of the frog 

 is thus the reverse of that which follows administration of adrenalin in the 

 frog, and both adrenal and pituitary autocoids in Fundulus, if Spaeth's 

 observations on the latter are correct. 



2. The melanophore stimulant of pituitary extracts is only slowly destroyed 

 by boiling with - 5 per cent. HC1 ; it is hence not identical with the pressor 

 principle, and in its slow destruction by acid hydrolysis agrees with the 

 oxytocic or uterine principle. 



3. That it is not identical with histamine is shown both by the inefficacy of 

 this drug to induce expansion of the melanophores, and by the readiness with 

 which it is destroyed by tryptic digestion ; it is not destroyed in a corre- 

 sponding manner by pepsin. 



4. That the pituitary stimulant acts directly on the dermal melanophores 

 rather than on the nerve endings is indicated by the failure of apocodeine, 

 atropine, curare, and cocaine, to abolish the reaction when administered in 

 doses which, on general grounds, would be regarded as sufficient to paralyse 

 all nerve endings. 



This research was carried out in Prof. McBride's laboratory ; acknowledg- 

 ment is made to Prof. McBride for his kindness in reading the MS., and to 

 Dr. H. H. Dale for generous assistance with reference to the literature. 



Cf. Introduction. 



