METAMORPHOSIS OF AMPHIBIAN ORGANS 285 



gills and skin, is controlled by one or more common centers, 

 by which the factor in question is produced. The fact however, 

 that certain important characteristics are common to the meta- 

 morphosis factor of each of these organs, encourages the belief 

 that we are dealing in each of these organs actually with the same 

 factor. The results shown by feeding thyroid and particularly 

 by the extirpation of innersecretory glands seem to support this 

 assumption. 



5. We have seen that the metamorphosis of the skin of Ambly- 

 stoma punctatum consists in the development of the yellow spots 

 of the adult. What has been said in the foregoing pages about 

 metamorphosis means then, that the mechanism which underlies 

 the development of the yellow spots contains one factor which 

 does not belong primarily to the specific properties of the skin, 

 but is formed at some place inside of the body of the organism, 

 at a certain time, and is carried into the skin secondarily. This 

 factor is not specific even for the individual within the species; 

 it is produced by every individual in the same way and there- 

 fore can be replaced for the skin of one individual by the body 

 of any other individual of this species, without changing the 

 final result. Furthermore: this factor is not specific even for 

 the species as it can be produced by another species also, as the 

 heteroplastic grafts have shown. Hence one of the most promi- 

 nent characteristics of this factor is its non-specificity, it is neither 

 specific for the organ nor for the individual nor for the species. 



In addition to this non-specific factor a second factor appar- 

 ently has manifested itself in the mechanism of the development 

 of the yellow spots — it is the factor responsible for the type in 

 which the yellow spots will develop. This factor seems to be 

 highly specific, as suggested by the constancy with which a 

 certain individual and even a certain piece of skin of this indi- 

 vidual will develop a particular type of spot, and apparently 

 lies in the skin itself. 



As pointed out above, the character of the yellow spots varies 

 greatly, and hardly two individuals can be found in which the 

 spots are alike. The spots vary in (1) the shade of the yellow 

 pigment, (2) the density of the yellow pigment (3) the size of 



