242 EDUARD UHLENHUTH 



It is well known that during metamorphosis the Amphibian 

 skin is subjected to a large number of changes, some of which 

 concern only the histological structure, while others are of such 

 a nature as to alter the external aspect of the organ also. It is 

 clear that we could not use the first ones, as the nature of our 

 experiments does not permit of killing the animals for examina- 

 tion. Among the external changes only the development 

 of the permanent coloration of the skin was used. As we shall 

 see it is possible to discern a number of definite stages within the 

 process of working out of the skin colors. The development of 

 each of these stages may, and according to what has been said 

 on page 1 of this paper actually does correspond to certain more 

 or less complicated internal physiological processes, which are 

 visible neither to the microscope nor to the naked eye; by means 

 of these processes a number of internal changes may be effected, 

 the final expression of which are those morphological changes 

 with which alone we will be concerned in this paper. The 

 nature of the internal changes connected intimately with the 

 production of the pigments may be important and so far as the 

 gathering of a certain kind of chromatophores at certain places 

 into single spots is concerned seem to be of particular interest, 

 but need not be discussed here. Concerning the action of the 

 physiological processes, certain features of their nature have been 

 disclosed by experiments similar to those to be reported and 

 these are described in former papers concerned with these 

 experiments. 



Before entering into a description of the development of the 

 skin coloration it should be mentioned that among the .Ameri- 

 can species of Salamanders available for our experiments, there 

 is none in which the development of the skin shows such a 

 definite step as is the appearance of the yellow spots in the 

 European species Salamandra maculosa. In Amblystoma punc- 

 tatum the development of the permanent skin patterns of the 

 adult is much more continuous and gradual; nevertheless as we 

 will see it is possible to divide the development of the yellow 

 spots of the adult animal into a number of well defined develop- 

 mental stages. 



