COELENTERATES AND PLANARIANS 209 



factor, such as a pull, can produce separation. Furthermore, nerve stimulation 

 may fail to be transmitted from one partner to the other. This incompatibility 

 between adjoining surfaces is also evident in the experiments of Peebles in 

 Hydractinia. When a piece of Pennaria was grafted on Tubularia, the 

 coenosarc united temporarily, but no union of the perisarc took place, and 

 after formation of the hydranths the pieces disintegrated. Similarly, the union 

 between Eudendrium and Pennaria was only imperfect and temporary. While 

 here homoiotransplantation may, at least in some cases, be perfect, in hetero- 

 transplantation the union of the coenosarc does not last, and if farther distant 

 species are used, injurious effects become still more noticeable. 



In general, buds develop, as H. D. King has shown, at the point of union 

 of two different kinds of organisms; these represent a mixture of the con- 

 stituents of both partners and thus constitute a chimaera. When the organis- 

 mal differentials of the two partners are markedly similar, the mixture is more 

 complete and the character of the tissue interaction differs from that seen 

 when the organismal differentials have less similarity. In the latter instance 

 parts of one organism have a tendency to penetrate as a connected mass into 

 the other, the constituents of both partners remaining more separate and dis- 

 tinct than when the organismal differentials are very much alike. In a very 

 interesting way the domination of one organism over the other, when they 

 differ in the constitution of their organismal differentials, has been shown in 

 the experiments of Goetsch and Issayew, who found that when two individuals 

 belonging to different species are united into one organism, budding often takes 

 place, the buds representing chimaerae of various kinds in which, however, 

 the constituents from one of the two species predominate. Issayew obtained 

 chimaerae also by cutting individuals from two different species into small 

 particles, which, when mixed, united to form one complete organism repre- 

 senting a mosaic of both partners. The union of Pelmatohydra oligactis and 

 Hydra vulgaris into a chimaera leads to a struggle between the constituents of 

 the two partners, in which the former gradually infiltrates and almost replaces 

 the latter; Pelmatohydra dominates and apparently only certain interstitial 

 cells of Hydra vulgaris remain preserved. The buds from such chimaerae may 

 be either Pelmatohydra or a mixture of both species. The remaining inter- 

 stitial cells of Hydra vulgaris are totipotent and may give rise to whole 

 organisms. 



The dominance of one species over another in heterotransplantation may 

 also become manifest in another way. If an excess of tentacles has been pro- 

 duced as a result of transplantations, the dominant species may determine 

 the number of tentacles which shall be absorbed and, in the end, the number 

 which is characteristic of the dominant species remains. We see that even in 

 this case of heterotransplantation the integrative factors, tending towards the 

 reestablishment of an organ equilibrium which accords with the predifferen- 

 tiation of the dominant species, are active. As a rule, that species which, in the 

 separate state is the more vigorous one dominates. We shall find also in 

 amphibians joined together in embryonal stages, a dominance of one partner 

 over the other, in accordance with the more rapid growth and greater size of 

 one of these species in the free-living stage. 



