METAMORPHOSIS OF AMPHIBIAN ORGANS 241 



which show that the mode of distribution by this flow of the 

 substances determines the development of a certain structure 

 at a certain place. 



And finally, certain experiments are in progress and have been 

 partly reported (Adler, Smith, Allen, Hoskins) by means of which 

 one single organ (Hypophysis, Thyroid) has been removed from 

 the Amphibian body, and it seems that the absence of this single 

 organ actually prevents or modifies metamorphosis in some way. 



From all this it seems that a study of the process of meta- 

 morphosis of the skin gains renewed interest and that the dem- 

 onstration of a metamorphosis factor acting on the skin in a 

 way similar to that of the metamorphosis factor involved in the 

 metamorphosis of the eye and the gills, would be of great im- 

 portance. For this reason a series of grafting experiments on 

 Amblystoma punctatum and Amblystoma tigrinum were per- 

 formed in the late spring of 1916. Special attention was paid 

 to the metamorphosis of the grafted skin but in several cases the 

 metamorphosis of the eye was also watched. 



II. CERTAIN MORPHOLOGICAL CHARACTERISTICS OF THE META- 

 MORPHOSIS OF THE NORMAL SKIN AND EYE IN 

 AMBLYSTOMA PUNCTATUM 



A. SKIN 



As can be seen from the foregoing pages and will better be seen 

 from the next chapter, the method of these experiments is such 

 as to compare the skin of different individuals as to the time at 

 which metamorphosis occurs or as to the rate at which develop- 

 ment progresses. This cannot be done unless we have certain 

 definite characteristics at hand which mark the time of the 

 entrance of the skin into each stage concerned in this compari- 

 son. Therefore it is necessary to outline here briefly the course 

 of the development of those characteristics which we have chosen 

 for the purpose of indicating the entrance into certain develop- 

 mental stages and to signify what characteristics we have de- 

 cided to be our indicators. 



