240 EDUARD UHLENHUTH 



morphosis of the gills a metamorphosis factor is involved, which 

 possesses the first characteristic of the factor governing eye- 

 metamorphosis. No experiments were made to examine the 

 three other factors. 



In 1913, several months after the author's first publication 

 on the subject in question, Weigl's paper on homoplastic and 

 heteroplastic transplantation of the Amphibian skin appeared. 

 Some of his experiments were made in exactly the same manner 

 as the author's eye-grafting experiments. To the facts obtained 

 by these experiments we will have to refer later on. The con- 

 clusions however, which Weigl pointed out in a chapter on meta- 

 morphosis, were entirely contradictory to our results. From 

 his experiments Weigl formed the opinion that the metamor- 

 phosis of the skin is a process of absolute self-differentiation of 

 this organ (Roux) 2 while we found on the contrary, that the 

 eye is unable to metamorphose unless it is furnished by the 

 animal's body with a certain factor or agent, which alone can 

 induce metamorphosis of the organ. If the conclusions of Weigl 

 were right, the theory that metamorphosis of the organs is en- 

 acted from a common center in the body would certainly lose 

 much of its interest and probability. 



Meanwhile however, (Judernatsch's experiments on feeding 

 tadpoles on thyroid, the first results of which were reported only 

 a short time before the writer had finished his first group of 

 experiments, have been repeated by several authors (Romeis, 

 Kahn and others) and it is certain that thyroid substance can 

 induce metamorphosis of the whole animal within a relatively 

 short time. This again suggests that in normal metamorphosis 

 the development of all organs, which take part in it, is governed 

 by one center and that metamorphosis of these organs is not a 

 process of self-differentiation. 



Furthermore J. Loeb, following the ideas of Sachs, in a recently 

 published book, has developed a theory according to which the 

 flow of certain substances plays a very important role in the 

 development of the organs. A large number of facts are quoted 



2 R. Weigl, p. 620. 



