SKIN TRANSPLANTATION IN FROG TADPOLES 395 



ACQUISITION OF MELANOPHORES BY WHITE GRAFTS ON A BLACK 



REGION 



1. Autotransplants 



It has already been remarked in this paper that white skin 

 transplanted from the venter to the back acquires black pigment. 

 Although the appearance of melanophores in such grafts varied 

 in autotransplants and homoiotransplants, yet the final result 

 was the same in both cases. An attempt was made to discover 

 how this condition was produced. The experiments of the first 

 group were autotransplants. In these cases the melanophores did 

 not appear until several weeks after the operation. The process 

 was very slow, and in three grafts was not completed at the end 

 of five months. The way in which it began was always the same. 

 Along the line of union between the graft and the surrounding 

 skin, the melanophores began to collect in large numbers, forming 

 a conspicuous black border. Very slowly this border of pigment 

 cells widened, becoming less dense as it covered more and more 

 of the graft. Sometimes the melanophores in the pigmented area 

 were arranged in such a position that their long axes were approx- 

 imately perpendicular to the nearest edge of the graft. This 

 produced the parallel arrangement illustrated by figure 22, al- 

 though this condition was never as prominent in autotransplants 

 as it was in homoiotransplants. Another arrangement was found 

 in some grafts, in which the melanophores were more numerous 

 along the four diagonals of the graft than between them. Dif- 

 ferences in size, as well as in the distribution, of the cells were also 

 evident. The largest melanophores were at the line of union, 

 and the smallest ones were found at the other hmit of the pig- 

 mented border. In this region also large and small masses of 

 melanin granules were abundant. Some of the largest masses 

 were about tht size of melanophores, but without the definitive 

 shape of such pigment cells. Occasionally, however, a large 

 melanophore of typical appearance could be seen among these 

 masses. Centripetal spreading continued until the whole area 

 was pigmented, giving to the graft a gray color. As time went 

 on, the shade of gray deepened, due to an increase in the number 



