METAMORPHOSIS OF AMPHIBIAN ORGANS 281 



Hence it is clear that the true relation existing between the body 

 of the individual and its organs cannot be studied unless atten- 

 tion is paid to age and developmental stage of both of them. 

 This can only be done if the age of each individual is carefully 

 registered and the developmental stage of each animal is recorded 

 in definite terms, and if a special series of experiments is devoted 

 to each of the ages and stages which play a role in the phenomena 

 under discussion. Unfortunately such data are missing in 

 Weigl's paper; it is impossible to form any conception of the 

 age and developmental stage of such larvae which Weigl calls 

 'young' larvae or 'old 7 larvae. If the experiments are made with 

 the, point in mind mentioned above, the results become uniform 

 and point to the conclusion that metamorphosis is not a process 

 of self-differentiation. 



Weigl's experiments may be briefly reviewed here in order to 

 show that they do not contradict the existence of a metamorphosis 

 factor. 



First: he found that pieces of skin which were grafted from 

 such larvae as he calls 'young,' to other 'young' larvae, always 

 metamorphosed simultaneously .with the host, which is in per- 

 fect accord with our own results, and Weigl himself expresses 

 this fact in terms that can hardly be brought into agreement 

 with his opinion that metamorphosis is a process of independent 

 differentiation, since he says that the graft becomes entirely 

 subjected to the influence of the host. 



Second : he found that pieces of skin grafted from young larvae 

 to old larvae metamorphosed some time after the host. He does 

 not say how young the 'young' larvae were in this case nor how 

 old the 'old' larvae were. If the 'young' larvae were only several 

 weeks old and the 'old' larvae old enough to metamorphose a 

 few days after the operation, this result also would be in accord- 

 ance with our own results. It does not mean, however that 

 there is any justification for the assumption that in these young 

 grafts metamorphosis occurred independently from the host's 

 body; it only means that the metamorphosis factor must act a 

 certain length of time before metamorphosis in the organ can 

 begin. . If the host metamorphoses before this time is elapsed, 



