386 WILLIAM H. COLE 



absorption cases and the rate of absorption are in direct propor- 

 tion to the degree of curvature. The mechanical stimulus due 

 to curvature, however, is effective only when the graft is com- 

 posed of plastic tissue, like tail skin. The compact tissue of 

 back skin is not able to adjust itself. The stimulus is probably 

 a tension of some sort, resulting from the healing process. An 

 appearance of stretching in grafts is very common. The tension 

 acting on the graft leads to a local disappearance of tissue. This 

 absorption in turn relieves the tension, and the graft shows no 

 further adjustment. It may be concluded, then, that the in- 

 hibition of the visual function in the frog tadpole by an opaque 

 graft does not effect any regulation of the graft. The adjust- 

 ment which does occur is purely a mechanical affair. There are 

 two possible reasons why no regulatory effect is seen. First, the 

 eye has lost the power, which it possessed during its organogeny, 

 of causing the overlying skin to become transparent. This loss 

 might be expected, since the eye has completed its development. 

 Secondly, assuming that the eye does act upon the overlying skin, 

 the latter has lost its power of response to such action. This 

 condition of the skin could likewise result from the completion 

 of development. In either case, the high degree of differentia- 

 tion existing both in the eye and the skin is the fundamental 

 reason why there are no effective interacting forces. The loss 

 of the power of regeneration, of regulation, and of various other 

 processes by an animal at certain periods of its development is a 

 familiar fact. It has been called the law of genetic restriction. 

 On this ground, the absence of any functional regulation in an 

 opaque graft over the eye of a frog tadpole is explained. 



LOCAL SPECIFICITY OF INTEGUMENT 



1 . Normal integument 



The arrangement of pigment cells in a frog tadpole's tail is 

 quite different from that seen on the back region of the animal. 

 In the skin of the back dermal melanophores and xantholeuco- 

 phores are very abundant, and, when expanded, they form an 

 intricate network of interlacing processes. These pigment cells 



