SKIN TRANSPLANTATION IN FROG TADPOLES 377 



b. Back skin. The second group of transplantations over the 

 eyes consisted of grafts of integument taken from the back region 

 of the same animal or from another animal. A square of skin from 

 the middle of the back was cut out and grafted over the eye in 

 the same way as with tail-skin grafts. The histories of such grafts 

 differ very markedly from the histories of tail-skin grafts, since 

 the former never show absorption or proliferation. The healing 

 perjod is essentially like that described for tail-skin grafts, but 

 the subsequent behavior varies according to the source of the 

 tissue. Autotransplants remain unchanged and enter the period 

 of equilibrium after about four days. The line of union is marked 

 by a denser mass of melanophores than elsewhere for several 

 weeks afterward. But in time this condition disappears and the 

 graft becomes indistinguishable from the surrounding skin (fig. 17). 

 Vision is thus permanently inhibited, since back skin is thoroughly 

 opaque. Neither increase in size nor any other indication of 

 growth is shown by such grafts, some of which were under ob- 

 servation for five months. Homoiotransplants of back skin over 

 an eye are not usually successful. When placed flat on other 

 parts of the body, they unite readily, but the healing period is 

 prolonged. This is probably due to the action of the homoio- 

 toxin upon the protoplasm of the graft, which has been shown by 

 Loeb to delay healing of grafts. When the graft is placed over 

 an eye the tissue is forced into a curved position by the convexity 

 of the eye. Because of the delayed healing and the curvature of 

 the new position, homoiotransplants of back skin are not able to 

 establish good union. Usually only one edge succeeds in becom- 

 ing attached, so that nearly all of the graft soon dies. But the 

 homoiotransplants which did unite completely had histories 

 similar to those of the autotransplants; they showed neither 

 absorption nor proliferation. 



The striking difference in behavior between tail-skin and back- 

 skin grafts may be due to the physical difference in structure. 

 Tail skin is mostly epidermis, with a very thin dermal layer, while 

 back skin has a well-developed dermis with many glands and a 

 dense fibrous layer. Therefore, back skin is thicker, more com- 

 pact and much less plastic than tail skin — facts that may well 



