COELENTERATES AND PLANARIANS 211 



restitution of a head in the headless part, although also in this type of hetero- 

 transplantation certain disturbances appear ; thus, distance actions which take 

 place in cases of homoiotransplantation, leading to reversal of polarity 

 (heteromorphosis) in the transplant or to the formation of buds in the host 

 under the influence of the transplant, do not occur. There is, therefore, under 

 these conditions an interference with the transmission of the regenerative or 

 organizer influence, which under other circumstances would have passed from 

 host to partner, or vice versa. For the most part, either an absorption of the 

 transplant takes place in these heterotransplantations, or the grafted head 

 separates from the host. We may then conclude that after heterotransplanta- 

 tion incompatibilities develop between the partners or between a transplanted 

 organ and the host at the point of union. This often leads to early separation 

 and, in addition, difficulties may possibly develop in the passage of active sub- 

 stances from one organism into the other. 



We must now inquire how far the reactions which have been observed when 

 we unite organisms belonging to different species or races in hydrozoa, can be 

 considered as due to differences in organismal differentials. There are two 

 circumstances which favor this interpretation: (1) The severity of these in- 

 compatibilities corresponds approximately to the distance of relationship be- 

 tween the parts which are joined together, and (2) the reactions after hetero- 

 transplantation seem to occur irrespective of the place where the two strange 

 organisms are united ; this fact suggests the presence of the same organismal 

 differential in all parts of the same individual. 



As stated above, while in general only heterodifferentials lead to noticeable 

 incompatibilities in hydrozoa, we cannot therefore conclude that individuality 

 differentials do not exist in these primitive forms. We must consider the 

 possibility that each individual within a race or species has its own individu- 

 ality differential, which differs from that of every other individual, but that 

 the incompatibilities which result from these differences between differentials 

 in the lower types of animals are too slight, in proportion to the resistance of 

 the affected tissues, for injurious agencies to become manifest. It is this rela- 

 tion between the degree of incompatibility, the resulting injurious reaction on 

 the one hand, and the resistance of the transplant, which might be expected to 

 determine the degree of disequilibrium arising from differences in the differ- 

 entials. If the individuality differentials are as yet only very slightly developed, 

 the incompatibility resulting from the union of parts of different individuals 

 may not become evident. On the other hand, it is possible, after all, that in- 

 dividuality differentials are not yet present in these primitive organisms. The 

 second alternative might even be the more probable one, because there is 

 reason for assuming that in young vertebrate embryos fully developed indi- 

 viduality differentials do not as yet exist. By analogy we may extend this con- 

 clusion also to adult individuals belonging to very primitive vertebrates. 



We must now return to a discussion of the conditions which maintain the 

 normal organism in a definite formative and functional equilibrium, and of 

 the similarities or the differences observed between higher and lower organ- 

 isms in this respect. In higher organisms such a formative equilibrium de- 

 pends, in part at least, on local conditions affecting the tissues; interactions 



