276 EDUARD UHLENHUTH 



ment IV; it has lost its yellow ring almost entirely, only a few- 

 yellow dots remaining. The grafted eye of experiment IV did 

 not reach this condition before the middle of November, when 

 it was photographed and painted again (fig. 17). Only a small 

 number of yellow dots were still present. 



Thus it seems impossible to overlook in these experiments a 

 definite time relation between the metamorphosis of the host 

 and that of the grafted organs, which evidently means that the 

 organs of A. punctatum cannot metamorphose before the host 

 does and that they are forced into metamorphosis as soon as 

 the host metamorphoses. The ability of one species to bring 

 into metamorphosis the organs of another species, is the result 

 which we wish most to emphasize among the facts disclosed 

 by this series of experiments. 



IV. DISCUSSION 



1. The experiments reported in this paper seem to throw some 

 light on certain fundamental principles upon which the mechan- 

 ism of the metamorphosis of the Amphibian skin is based. 



Let us consider first the experiments of series XXV in which 

 from the head of one individual each of the two halves of the skin 

 was grafted to another individual of the same species and the 

 rate of development of the yellow pigment spots of the adult 

 was compared in each graft with the rate of the same process 

 in the respective host and the other graft. Under normal 

 conditions, i.e., if the two pieces of skin had remained with the 

 individual to which they originally belonged, both pieces would 

 have developed the yellow spots at the same time and would 

 have metamorphosed simultaneously with each other. If all 

 the factors necessary to the development of the yellow spots 

 were in the skin itself, i.e., if this phenomenon were actually a 

 process of self differentiation as Weigl claimed it to be, there 

 would be no reason why two pices of skin of the same animal 

 should not develop simultaneously with each other even after 

 they have been separated from this animal and transferred to 

 different individuals. 



