326 



Dr. L. T. Hogben and Mr. F. K. Winton. 



The result of the experiment indicates that the melanophore stimulant of 

 pituitary extracts is not destroyed by pepsin or HC1 - 2 per cent., but that 

 it is completely and rapidly destroyed by trypsin. This is in accordance with 

 Dale's work on the uterine and pressor principles. 



As the foregoing experiments show, the melanophore stimulant, like the 

 other pituitary autocoids, is not destroyed by boiling ; it is immediately 

 destroyed in the presence of a small quantity of H 2 2 . One other point 

 deserves mention in this connection to correct a possibly erroneous interpreta- 

 tion of results previously published by one of the authors. It was stated 

 (Huxley and Hogben, 1921) that pituitary feeding induces melanophore 

 expansion in salamander larvae. In view of the destruction of the melanophore 

 stimulant by trypsin, it seems highly probable that the effect was not produced 

 as in the case of thyroid feeding by absorption of the autocoid vid the digestive 

 system, but by traces in solution in the medium, acting through absorption 

 by the skin. 



6. Mode of Action of the Melanophore Stimulant. 



The effect of injection of pituitary extracts on the intact animal might be 

 interpreted in at least four different ways : — 



(a) Local effect on the circulation (vasomotor). 

 (&) Central stimulation. 



(e) Action on nerve-endings in melanophores. 

 (d) Direct action on the melanophores. 



The first two are dismissed in view of the fact that, with appropriate 

 preparations, pituitary extracts can be made to exert their characteristic 

 reaction upon isolated skin. The second is also excluded by the possibility 

 of producing the same reaction after destruction of both spinal cord and 

 brain. To discriminate between (c) and (d), two methods may be adopted : 

 first, the injection of pituitary extract after the complete degeneration of the 

 nerve supply of the skin in a particular area ; and, secondly, injection after 

 paralysis of the nerve-endings which supply the melanophores by means of 

 the usual drug series. The first is the more critical, and it is hoped to carry 

 out this method later. The second is open to the objection that the 

 evidence is based on the analogy of the operation of drugs on plain muscle. 

 As far as the results of experiments on these lines permit legitimate 

 inference, it would appear that the action of the infundibular melanophore 

 stimulant is dh*ect and independent of any nerve-endings. 



The following re-agents were used for paralysis of different types of 

 nerve-endings : cocaine (afferent), curare (spinal efferent), atropine (para- 

 sympathetic), and apocodeine (sympathetic). The use of cocaine was 



