TRANSPLANTATION OF PIECES OF TISSUE 247 



by means of the circulation. However, we have already seen that incom- 

 patibilities between organismal differentials may become manifest even with- 

 out injurious effects being transmitted through the blood. In Harrison's 

 heterotransplantations of tails of anuran amphibians, atrophy and degenera- 

 tion often set in within a few weeks after grafting, although some parts of the 

 transplant could survive. But it is possible that vascular changes, interfering 

 with the circulation of the blood, and caused presumably by the incompati- 

 bility of the organismal differentials of host and graft, were at least partly 

 responsible for the degenerative conditions that occurred in the experiments 

 of Braus. 



It is of interest to compare with these transplantations of small pieces the 

 results obtained in the joining together of larger parts of larvae of Rana 

 esculenta and Bombinator. In Born's experiments such combinations lived 

 only for three weeks, but, according to Braus, they may persist longer under 

 favorable conditions. It seems, then, that in both cases after heterotrans- 

 plantation incompatibilities developed, which caused a cessation of growth. 

 We may conclude that certain, not well defined, growth factors may be potent 

 even in heterotransplantations between amphibian larvae, and that the sub- 

 stances circulating in the body fluids of the host which regulate the growth 

 processes, may be independent of organismal differentials, as are also other 

 growth-regulating substances, such as certain hormones, which apparently 

 do not carry organismal differentials. As in the case of tumors, we must dis- 

 tinguish from these hormone-like, growth-regulating substances, other growth- 

 determining factors which are inherent in the transplanted tissue itself, and 

 which continue to assert themselves even in a heterogenous soil, provided the 

 heterotransplantation does not preclude the life of the transplanted tissues. 



As to the relation of these inherent growth substances to the organismal 

 differentials, these experiments do not give any indication. As stated above, 

 between Amblystoma tigrinum and Amblystoma punctatum, limb, and also 

 eye, can be readily exchanged, and both of these organs may then continue 

 to live. Normally, Amblystoma tigrinum reaches a greater size than Amblys- 

 toma punctatum and the experiments of Twitty and Schwind indicate that the 

 transplanted extremities retain essentially the characteristics, as to growth 

 energy, of the species or race from which they are derived. This may per- 

 haps be due to the fact that the growth factors inherent in the transplanted 

 tissues dominate over extraneous growth factors, which are transmitted to 

 them through the circulation of the host. Similarly, Burns and Burns found 

 that heterotransplantation between larvae in these two species of Amblystoma 

 succeeds if young stages are used for this purpose, and that under these 

 conditions both partners retain their intrinsic growth momentum. Likewise, 

 the specific stimulus to metamorphosis is not transmitted from one partner 

 to the other, or if transmitted, is ineffective in these transplantations, but the 

 sexual characters of one partner may be influenced by those of the other. 

 Also, in the case of heterotransplantation of eyes the transplanted organ 

 retains the growth energy inherent in the donor tissues. In some cases the 

 inherent growth energy of the donor tissue may be the deciding factor only 



