SKIN TKANSPLANTATION IN FROG TADPOLES 355 



interpretation of this relation. It is held, on the one hand, that 

 function is causal to form and structure, while on the other, that 

 it is resultant. 



In line with the studies on the production of transparency in the 

 ectoderm over the young eye, it was suggested to the writer by 

 Prof. H. W. Rand that the experimental inhibition of vision by 

 grafting opaque skin over the eyes of frog tadpoles, long after 

 the eyes had been fully formed, might contribute evidence as to 

 any interacting influences between the graft and the eye. Pos- 

 sible results of such an experiment are, first, that the graft may 

 become transparent by a thinning process and by a loss of its 

 pigment cells, thus repeating the embryological history of the 

 ectoderm over the eye. This would indicate that there is a direct 

 relation between the visual function and the production of trans- 

 parency. Secondly, the transplant may remain unchanged, in- 

 hibiting vision permanently. Such a result would show that the 

 skin had lost its power of becoming transparent or that the eye 

 exerted no influence upon the graft. Thirdly, the transplant 

 may be completely absorbed or thrown off, thus restoring vision 

 and strengthening the causal-function theory. In my early 

 experiments, which consisted of grafting tail skin over the eyes 

 of frog tadpoles, the first and third possibilities were not realized; 

 the second one sometimes was, while the majority of the trans- 

 plants exhibited an entirely unexpected kind of adjustment or 

 regulation. This adjustment consisted of the absorption of part 

 of the graft in such a way that light could reach the eye directly. 

 When this was accomplished, absorption ceased. This discovery 

 suggested that the function of the eye might be causal to the 

 adjustment process. Since no satisfactory explanation could be 

 formulated from the results, further experiments were planned 

 to answer the question. Is the absorption process really initiated 

 and controlled by the interruption of vision? During the greater 

 part of the investigation this question could not be definitely 

 answered in the negative, and much evidence pointed to its 

 afhrmation. Finally, from the results of check experiments, in 

 which 'artificial eyes' were used beneath the grafts in other parts 

 of the body, proof was obtained that the visual function does not 



