282 EDUARD UHLENHUTH 



the grafted organ will metamorphose somewhat later than the 

 host. Furthermore as repeatedly pointed out in this article, 

 only one factor necessary for metamorphosis is furnished to the 

 graft from the host, while the others must be contained or de- 

 veloped by the graft itself and as long as they are not — which 

 may be the case in very young animals — the skin will not re- 

 spond to the action of the metamorphosis factor. Whether we 

 call the factors contained in the organ itself 'structure' as Weigl 

 did, or whether we imagine them to be something else, does not 

 alter the fact that the final result is brought about with the 

 aid of the one factor contained in the body of the host, as is 

 shown in Weigl' s experiments by the fact that the young organs 

 after having been grafted to old larvae, metamorphosed earlier 

 than they would have done when left with the young larvae 

 from which they were taken. Metamorphosis therefore, mani- 

 fests itself as a process of 'dependent differentiation.' 



Third: Weigl reports one experiment in which the skin of a 

 larva which he calls 'young' was grafted to another larva of the 

 same species (Salamandra maculosa) ; the skin graft metamor- 

 phosed six months before the host was photographed and the 

 photograph shows distinctly that the host is still a larva. This 

 case could hardly be explained on the basis of metamorphosis 

 being induced by a factor external to the organ, for such a factor 

 could neither have been carried into this piece of skin from the 

 animal to which it first belonged, since this animal was a young 

 larva according to Weigl, nor could it have been furnished to it 

 by the host, since the host was still in larval condition. The 

 fact that among the many grafting experiments which I had 

 occasion to observe such a result was never obtained makes it 

 appear doubtful whether it could be obtained at all, and there 

 are no data even in Weigl's paper itself which explain why he 

 should have obtained such a result only in one case and not in 

 other cases. We have seen that this result was obtained only 

 when the larvae from which the graft was taken were so old as 

 to metamorphose a few days after operation, and this seemed to 

 be in consequence of the metamorphosis factor being already 

 developed in such very old larvae and carried into the organ. 



