268 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



by the taxonomic relationship between host and transplant, the correspond- 

 ence between toxicity and taxonomic relationship is not perfect. There are 

 some species which are especially poisonous for Triton, although they may 

 not be farther removed in relationship than other less poisonous species. (3) 

 The relative rapidity of embryonal development in host and transplant. If in 

 the transplant the development takes place very rapidly, then sufficient time 

 may not be available for the organizer action to become effective. 



In general, under the best of conditions xenogenous transplantations are 

 difficult. In many cases the transplant is expelled or absorbed after it has been 

 lying for some time in the deeper parts of the host as an apparently inert, 

 though living tissue ; but it may be possible to transplant presumptive epidermis 

 of Bombinator into the ectoderm of Triton and have it develop here into skin; 

 but such skin soon becomes thinner and gradually it disappears. After trans- 

 plantation into deeper parts of the host it is either discarded or gradually 

 absorbed, although in this position the graft may at first act like an interplant. 

 For a short time it may develop and possibly even be effected by the organ- 

 izers of the host, producing mesodermal structures. But union does not take 

 place between host and transplant and chimaerae do not form. 



If presumptive medullary plate is transplanted from Bombinator into 

 Triton, two medullary tubes may develop in the host ; one is from the trans- 

 plant itself, the other, originating in the host, is determined by the transplant 

 acting as an organizer. But under these conditions interesting differences may 

 appear in the further fate of these new formations, from those observed after 

 heterotransplantation, owing to incompatibilities between the organismal 

 differentials of the two tissues. While the medullary tubes arising after hetero- 

 transplantation may coalesce into one single organ, those arising after xeno- 

 transplantation are unable to do so ; at best there may be a temporary union 

 between the two medullary formations, which is followed secondarily by a 

 separation. A difference between the behavior of hetero- and xenogenous 

 structures was found also in mesodermal formations. Here the difficulties in 

 the union of xenogenous structures may be still greater than those observed 

 in ectodermal tissues. While two medullary plates or tubes of xenogenous 

 origin may exist side by side, with mesodermal tissues single organs develop 

 either from the host or from the transplant, the one which develops first 

 apparently suppressing the other. In the case of mesodermal chordae, hetero- 

 transplanted organs may not unite with the analogous host organs. Here we 

 find, then, incompatibilities similar to those which are observed in the union 

 of distantly related eggs or young embryos in echinidae and Ascaris, or in 

 heterotransplantations in adult hydrozoa and planarians. In all these instances 

 the character of contact mechanisms, which presumably is contingent upon 

 the mutual suitability of contact substances, primarily determines the possi- 

 bility and durability of coalescence. As we have seen previously, the com- 

 pleteness of the union depends, at least in some instances, upon the lack of 

 regenerative growth processes at or near the point of contact, and this again 

 is determined by the relationship of the two organismal differentials which 

 interact. 



