METAMORPHOSIS OF AMPHIBIAN ORGANS 287 



is either absent from the skin of this individual, or if present is 

 of such a specific type as to permit development of the spots 

 only to a certain decreased amount and after a long time. This 

 result was actually obtained in the group of experiments under 

 discussion. 



Quite similar results were furnished by the experiments of 

 Weigl and several other investigators who studied the behavior 

 of the transplanted skin of Amphibians. 



Comparing this second factor with the first one, we see that 

 its chief characteristic is an apparent specificity. It is specific 

 for the organ itself, because it cannot be furnished to it by the 

 rest of the body, and it is specific for the individual because it 

 cannot be replaced by the corresponding factor of another 

 individual. 



It thus appears that two factors, easily distinguishable, have 

 been revealed as involved in the process of the development of 

 the yellow spots. One of them is a non-specific factor, the other 

 a specific one. The first one, it seems, may be identical with a 

 chemical substance such as a hormone ; the nature of the second 

 one is entirely unknown, but in common terms it might be 

 called a 'specific structure.' 



I should not have expounded these facts at such length if I 

 did not think them particularly apt to throw some definite light 

 upon the meaning of what we call 'specificity.' 



From the experiments reported here, we might well form the 

 opinion that each individual of Amblystoma punctatum carries 

 in its skin a specific structure, which is predetermined and ac- 

 cording to which the yellow spots of one individual must develop 

 certain definite characteristics different from those of other 

 individuals in which the corresponding predetermined specific 

 structure was different. This structure is unchangeable as 

 is also the type of the spots which develops from this structure; 

 and this is what is ordinarily termed 'specificity.' 



The following instance however, shows that these conclusions 

 would have been premature. In the course of certain experi- 

 ments not yet published, a series of A. punctatum larvae were 

 fed on thymus gland and controlled by another series kept on 



