METAMORPHOSIS OF AMPHIBIAN ORGANS 289 



3. The result was : 



a. The skin grafts developed the network and the yellow spots 

 not simultaneously with each other as they would have done if 

 left with the animal from which they were taken, but simul- 

 taneously with the host; they metamorphosed only if the host 

 did. 



b. This resulted in the establishment of a more or less con- 

 siderable difference in the time of development of the adult 

 characteristics between the two pieces of skin, provided such a 

 difference existed between the respective hosts. The difference 

 ranged from seven days to five months with regard to the net- 

 work development, and from three days to four months with 

 regard to the development of the yellow spots. Metamorphosis 

 of the skin graft was retarded or accelerated by the host. 



c. The grafted eyes showed a similar time relation with regard 

 to the breaking up of the yellow ring. 



4. Pieces of skin and eyes, which were grafted from larvae of 

 Amblystoma punctatum to larvae of Amblystoma tigrinum, not 

 only metamorphosed also on this host of a foreign species, but 

 showed, with regard to metamorphosis, the same time relation 

 as on a host of the same species. 



5. Individual types of the yellow spots of the skin of one 

 individual remained unchanged even when the skin was grafted to 

 an individual whose skin developed yellow spots of another type. 



6. From these facts it is evident that at least two factors are 

 involved in the mechanism of the development of the yellow 

 spots of Amblystoma punctatum: 



a. One factor, which is responsible for the kind of yellow spots 

 that will develop, and which is contained in the skin itself; 



h. Another factor which may be called 'metamorphosis fac- 

 tor 1 and which may be characterized as follows : 



1. It is necessary to start the process of the development of 

 the yellow spots; i.e. metamorphosis of the skin; 



2. It is not contained in the skin; but produced by the body 

 or some particular organ of the organism; 



3. It is non-specific ; 



4. It may be identical with an agent like thyroid-substance. 



