SKIN TRANSPLANTATION IN FROG TADPOLES 365 



3. Proliferation of the transplants 



Only those grafts which were taken from the tail showed any 

 growth, i.e., increase in size, of tissue. In some cases the new 

 tissue formed was irregular in shape and bore no resemblance to 

 a tail. In other cases growth resulted in the formation of a 

 miniature tail. In order to avoid any intimation that the growth 

 of tail-skin grafts was an attempt of the tissue to form a new 

 tail or a new body, the word proliferation is used. Proliferation 

 as used here means merely the formation of new tissue by the 

 transplant. The maximum amount of new tissue varied in 

 individual cases from one-tenth of the transplant to more than 

 twice its size (fig. 16). It was not determined what the factors 

 are which inhibit further growth when the maximum is reached. 

 They must be similar to those which halt the growth of all 

 normal tissues and organs. Whatever the cause, the fact that the 

 growth of the transplants is limited is no more remarkable than 

 the limitation of the growth of all normal tissues in any organism. 



The rate of proliferation is slightlj^ affected by the amount of 

 illumination. In the transplants kept in darkness proliferation 

 began earlier, proceeded at a greater rate, and came to an end 

 sooner than in the transplants kept in light. Under either condi- 

 tion the final result was the same, due to the longer period of 

 growth in those exposed to light. Another difference was that 

 the new tissue formed in darkness was more irregular or wrinkled 

 than that formed in light, possibly because of the greater rate of 

 growth. These differences were never marked, but the observa- 

 tion of a large number of cases proved their existence. 



In a transplant showing the maximum amount of proliferation 

 the original anterior and posterior ends had produced more new 

 tissue than the other two edges, regardless of the orientation of 

 the graft. In twelve cases proliferation was restricted to the 

 originally anterior end, and in twelve others to the originally 

 posterior end. In twenty-two grafts both ends proliferated 

 equally, forming an elongated diamond-shaped mass (fig. 16). A 

 tail-skin graft, therefore, retains its anteroposterior polarization, 

 both ends being able to proliferate new tissue. Lateral prolifera- 



