TRANSPLANTATION OF PIECES OF TISSUE 257 



However, while thus a great similarity exists between the successive changes 

 in the phylogenetic and ontogenetic series as far as the development of the 

 organismal and organ differentials is concerned, there are also some very 

 important differences in these two series. In phylogenetic evolution we have 

 to deal with changes in the genetic constitution of the organisms as the basis 

 for the corresponding changes in organismal differentials and in individuality, 

 and in differentiation and fixity of organs and tissues. On the whole, we can 

 trace the relationship between different species of animals, one species de- 

 veloping from the other in an apparently connected series and each possessing 

 its own kind of organismal differentials; the later species show a greater 

 complexity and fixity in the organismal differentials than the preceding ones 

 and, accordingly, also a greater differentiation and fixity of the organ and 

 tissue differentials. 



This same relationship obtains also if we compare the corresponding 

 ontogenetic stages in the different classes and species of animals; but in 

 ontogenetic development there is, as far as we know at present, in the con- 

 secutive stages of development throughout, an identity in the genetic consti- 

 tution of cells and tissues in the same individual and species. And if in the 

 series of ontogenetic stages likewise a development of the organismal differ- 

 entials takes place, it is one from the precursor predifferential stage in the 

 fertilized egg to the mature organismal differentials in the adult organism. 

 The genetic basis of the organismal differentials remains unchanged in all 

 these successive stages. We have then to deal, during embyronal life, with a 

 development of the organismal differentials that is parallel to the develop- 

 ment of organ and tissue differentials. In both, the genetic basis is already 

 present in the egg, and notwithstanding this sameness of the genetic consti- 

 tution throughout the series of successive ontogenetic stages, the constitution 

 of the tissues as well as of the organismal differentials changes progressively. 

 With this increasing development of organismal differentials out of their 

 precursor stages or predifferentials, the mutual compatibility of tissues pos- 

 sessing different organismal differentials decreases. In testing by means of 

 transplantation the effect of the increasing differentiation of tissues and 

 organs and their differentials on the plasticity of organs, the adaptability of 

 the organs and tissues to new environments, and the mutual interactions 

 between the grafts and host tissues, we introduce at the same time a second 

 variable in this experiment, namely, a change in the character of the organis- 

 mal differentials which also have progressed in the direction from their 

 precursors to more mature differentials in the course of embryonal develop- 

 ment. In observing the effects of the new environment on organs and tissues 

 we may thus have to deal with a summation of effects of both the changes 

 in organ and in organismal differentials. It is conceivable that likewise during 

 the process of regeneration in the earliest stages the precursors of organismal 

 differentials are present in the regenerating tissues, and that with progressive 

 regeneration the precursors of the organismal differentials present in ordinary 

 tissues and in totipotent cells, mature into the fully adult organismal differen- 



