HYBRIDIZATION AND TRANSPLANTATION 331 



restrictions, the existence of a parallelism between the transplantability of 

 tissues and the hybridizability of the organisms from which the tissues are 

 derived. 



A much more complete correspondence between the effects of hybridization 

 and transplantation was observed in the more recent experiments of von 

 Ubisch, who used, however, not adult organisms for transplantation, but early 

 embryonal stages of echinoderms. When he transplanted the micromeres, 

 which give origin to the skeleton, from one species, or even from one order, 

 into another one in which the character of the skeleton was different from 

 that of the first species or order, he observed the formation of an intermediate 

 skeleton in the plutei derived from these chimaerae ; this intermediate condi- 

 tion may represent either a mosaic of the skeletons of host and donor, or a 

 still more perfect combination. If hybrids were produced between the same 

 orders or species which were used for grafting, the hybrids developed a skele- 

 ton which was similar to that which developed in the corresponding chimaerae 

 following transplantation. It may be assumed that the nuclei of the two indi- 

 viduals which give origin to the third individual largely determine the results 

 in both hybridization and in the formation of chimaerae through transplanta- 

 tion; and although in the hybrid every nucleus contains both maternal and 

 paternal material, while in the chimaerae some cells have only nuclei of the 

 host and others only nuclei of the donor, still, in hybrids and chimaerae the 

 nuclear material from both parents, or from both host and donor is present. 

 This might explain the similarity in the results of transplantation and hybridi- 

 zation. However, whereas the chimaerae contain cytoplasm of both species, 

 the hybrid contains only the maternal cytoplasm. Therefore the results of 

 reciprocal hybridization may differ greatly, because the cytoplasm, which is 

 present only in the female sex cell, differs in reciprocal crosses, whereas in 

 the case of chimaerae, since both parents contribute cytoplasm, the cytoplasm 

 and therefore also the results of reciprocal transplantations are the same. But 

 this explanation may hold good only for transplantation of very early em- 

 bryonal material ; we have seen that in further developed organisms the results 

 of reciprocal transplantations may differ. 



In the primitive organisms employed in von Ubisch's experiments, the cells 

 and tissues were still very plastic and they possessed the precursors of, rather 

 than the fully developed organismal differentials, facts which may account 

 for the fargoing parallelism found in this instance between the results of 

 hybridization and transplantation, while such a parallelism is very much less 

 complete in experiments in which adult tissues are used. 



Concluding Remarks 



From the experiments on which we have so far reported, it seems to follow 

 that organisms in general represent organismal equilibria which in the case 

 of the most differentiated organisms may be autogenous; this means that all 

 the various constituent parts of organisms possess in common certain chemical 

 characteristics, which differ from those of all, or almost all, the other organ- 

 isms, and which prevent a tolerance for contacts between tissues derived from 

 strange organisms and, instead, cause reactions of aggression or defense. 

 This applies not only to complex metazoa, but also to certain free-living cells. 



