ORGANIZERS AND TISSUE DIFFERENTIATION 269 



An organizer effect in case of xenotransplantation may even be demon- 

 strable if the organizer action takes place at somewhat later stages of em- 

 bryonal development. Thus the ectoderm of the gill area can, to a certain 

 extent, transform ectoderm from other than the gill region of a xenogenous 

 gastrula into gill tissue. 



We may then conclude that the organizer can, in some instances, continue 

 to function, but usually only in a very limited way, if very distant organismal 

 differentials interact with each other. Moreover, it can be shown that also 

 the factors which determine self-differentiation leading to further develop- 

 ment may still act after xenotransplantation of an embryonal piece of tissue. 

 Thus, according to Bytinski-Salz, anuran presumptive mesoderm after 

 xenotransplantation, may differentiate into chorda and musculature, presump- 

 tive epidermis into epithelium of the skin. 



Mangold and Seidel succeeded in joining together early stages of segmen- 

 tation of Triton eggs belonging to the same species; in some cases a single 

 homoiogenous organism resulted from this combination, in other cases two 

 or more organisms developed. Mangold found that also union of heterogenous 

 Triton eggs in the two-cell stage of segmentation may succeed, but the number 

 of single organisms which resulted was smaller than after homoiotransplanta- 

 tion. After heterogenous union various organs which developed could contain 

 constituents of both species, which functioned without any antagonistic reac- 

 tions becoming manifest. However, as stated above, even under these condi- 

 tions various abnormalities developed in the case of chorda. It is these abnor- 

 malities observed in heterogenous early embryonal combinations, which sug- 

 gest that noticeable differences exist also in the character of the precursors 

 of heterogenous organismal differentials, and although such differences usually 

 do not become evident, they may lead to incompatibilities under certain un- 

 favorable conditions. 



In combining heterogenous parts in adult individuals belonging to different 

 species in primitive classes of invertebrates, we have noticed that it is usually 

 the larger piece which dominates over the smaller piece. Similarly after trans- 

 plantation of small parts of embryos it is the larger host which is usually the 

 dominating partner, the xenotransplant being, in most cases, either discarded 

 or destroyed ; but if the transplant belongs to a species particularly toxic for 

 the host, the latter may be injured and ultimately killed by the transplant. 



If thus xenogenous transplantations may succeed in amphibia and organizer 

 effects be exerted, these effects become manifest after a relatively short time 

 of interaction between the two strange tissues, a period too short perhaps for 

 the manifestation of incompatibilities between the organismal differentials. 



We have already referred to experiments of Spemann which showed that 

 under the influence of heterogenous inductors the receptive tissues undergo 

 differentiations into organs which are in accordance with the specific organ- 

 forming potencies of the organizers ; yet at the same time the organs and 

 tissues which do develop show the species characteristics of the recipient 

 tissues. 



Some very instructive experiments of a similar nature, illustrating the 



