METAMORPHOSIS OF AMPHIBIAN ORGANS 249 



left with animal A the skin of both halves of the head would 

 have metamorphosed at the same time ; namely : at the time when 

 the factors necessary for the metamorphosis of the skin after 

 they have developed in the body of animal A. and have been 

 carried into the skin, have acted in the skin a sufficient length 

 of time to cause the physiological changes necessary to produce 

 the morphological changes of the skin described in the preceding 

 chapter. But removed from the animal A and separated from 

 each other — one half having been grafted to X, the other half 

 to animal Y — they would not metamorphose simultaneously 

 with each other but simultaneously with their respective new 

 hosts and hence at different times; provided of course, that 



A 



Y 



metamorphosis of the skin followed the principles observed in 

 the metamorphosis of the Amphibian eye and gills. 



To use the skin of the head for grafting offers a special ad- 

 vantage. As pointed out in Chapter II, in studying metamor- 

 phosis of the skin of Amblystoma punctatum we must choose 

 some definite characteristic of the skin. For certain reasons 

 explained in Chapter II the appearance of the yellow spots was 

 found to be the most convenient time-mark. In the larvae, 

 however, where of course no spots are developed yet, it is im- 

 possible to foresee on what parts of the skin of the body such 

 spots will develop and as the distribution of the yellow spots is 

 subjected to great variation there would be little chance of hav- 

 ing selected for grafting a piece of skin that would finally develop 

 a spot. But this is different for the head, as in the majority 

 of animals one or two yellow spots will develop on the skin above 



