TRANSPLANTATION OF PIECES OF TISSUE 245 



pendent transformation of skin into lens tissues, as well as the production 

 of lens under the influence of the eye vesicle. 



As to the length of time during which such homoio- and heterotransplants 

 of skin remain alive, no systematic studies seem to have been made. However, 

 that the heterodifferential may after all assert itself is indicated by the 

 experiments of Filatov, in which larval skin of Bufo was grafted over the 

 eye of Rana esculenta. The lens developed from the transplant but subse- 

 quently it degenerated. Perhaps a certain length of time was required for the 

 cumulative action of the heterodifferential to become apparent. 



Somewhat analogous conditions exist also in other instances. Thus, Ekman 

 found that in larvae of Bombinator the ectoderm from the heart and kidney 

 regions, but not from other parts of the body, if transplanted to the gill region 

 is able to produce gills. There appears to exist a varying degree of rigidity 

 of the tissue differentials in analogous tissues at corresponding periods of 

 embryonal development in different species, and in addition, the tissue or 

 organ differentials may be specialized to a different degree in different areas 

 of the same individual. 



Whether these differences in the degree of plasticity of the skin are in any 

 way related to its transplantability and sensitiveness to strange organismal 

 differentials seems not to have been determined. But it is quite obvious from 

 these investigations that the tissue differentials may be graded in a much 

 finer way than is apparent from the manifest structural characteristics of the 

 tissues. That this is true also of tissues of the adult organism follows, for 

 instance, from our studies of the varying ability of connective tissue in dif- 

 ferent parts of the sex tract to produce placenta. 



In general, evidence is lacking that in transplantation in amphibian larvae 

 the individuality differential plays any particular role. In urodele, as well as 

 in anuran larvae, skin, extremities, tailbuds, eyes and other organs can be 

 readily homoiotransplanted. However, under certain conditions individuality 

 differentials may, after all, produce a certain effect ; thus, according to Hell- 

 mich, in anuran larvae a homoiotransplanted limb may heal in, but subse- 

 quently the transplant, ceasing to grow, shrinks and becomes necrotic. Other 

 effects of the individuality differential on transplantation in anuran larvae 

 will be discussed later. 



In urodele larvae heterotransplantation succeeds more readily than in 

 anuran larvae. For instance, between Amblystoma punctatum and Amblys- 

 toma tigrinum extremities can be readily exchanged. Likewise, transplantation 

 of extremities from larvae of Triton taeniatus to Salamandra maculata, and 

 other similar heterotransplantations, may be successful. However, this is true 

 only of transplantations in larvae. In metamorphosed urodeles, even after 

 autotransplantation the transplanted limbs are readily cast off, an effect which 

 must be due, however, to other factors than organismal differentials. Simi- 

 larly when, according to Detwiler, autotransplantation of a limb in larvae of 

 the urodele Amblystoma succeeds better than homoiotransplantation, this 

 difference in all probability does not arise from the direct injurious influence 

 of strange individuality differentials on the grafted tissue, but from secondary 



