SKIN TRANSPLANTATION IN FROG TADPOLES 387 



are more or less evenly distributed throughout the skin of the 

 back. Although there are distinct variations in pigmentation 

 between R. catesbeiana and R. clamitans, the foregoing state- 

 ments hold true for both species. In the skin of the tail the num- 

 ber of dermal pigment cells is very much smaller than in 

 that of the back. Under the binocular microscope individual 

 cells may easily be seen, even during extreme expansion. There 

 may be occasional collections of several melanophores and 

 xantholeucophores within a small area, in which the limits of 

 individual cells are obliterated. But in general the cells are 

 distinct. In R. catesbeiana the masses of melanophores form 

 definite rounded markings and are designated as specific charac- 

 ters for that species. In R. clamitans the spots have very irreg- 

 ular outlines and are not so dense in color and are inconspicuous. 

 The epidermal melanophores of both back and tail are essentially 

 alike, but differ slightly in distribution. On the back they are 

 more numerous than on the tail. As a result of these conditions, 

 the two kinds of skin bear characteristic markings or pigment 

 patterns which easily distinguish them from each other. In the 

 normal tadpole each region continues to produce skin of its own 

 kind. If an area is denuded of integument on the back or the tail, 

 it is quickly covered by new integument of the original type. Where 

 is the mechanism that controls this persistent production of one, 

 and only one, kind of skin? Is it located in the skin itself or in 

 the animal as a whole? Evidence on this question ought to be 

 obtained by transplanting from one region to another and ob- 

 serving what changes, if any, take place. The experiments con- 

 cerned with the adjustment process afforded an opportunity to 

 study such grafts. After they had been examined, other auto- 

 transplants were made. Back skin was grafted on to the tail 

 and belly skin on to the back. The behavior of homoiotrans- 

 plants from the same regions was also studied. 



2. Autotransplants 



Tail skin grafted on to the back is conspicuous because of the 

 difference between the pigment patterns of the two types of skin. 

 Figures 12, 14, 15, and 17 illustrate such grafts. The visible 



