COELENTERATES AND PLANARIANS 213 



equilibrium may, in certain instances, in hydroids as well as in planarians, 

 lead to the separation of autogenous parts of an individual which have been 

 present in excess, and which have become superfluous. Here it is evident that 

 regulatory, integrative mechanisms of an unknown nature constitute the 

 primary process, and this is followed by the creation of a wound surface as a 

 secondary effect ; the latter is, therefore, caused by the action of these integra- 

 tive mechanisms ; (4) there are indications that the mechanisms mediating the 

 maintenance of an individual as an equilibrated system are not so well trans- 

 mitted at the points of union between heterogenous partners. We may expect 

 in this case regulatory processes to be set in motion, leading to attempts at new 

 integrations of individuals, with the resulting separation of the incompatible 

 parts. 



We have seen that an individual hydrozoon can be divided, and that each 

 part can give origin to a complete organism. The degree to which divisibility 

 can be carried depends on whether such particles are kept in their normal 

 medium free from contact with other individuals, or whether they are trans- 

 planted to another organism ; if transplanted, the antagonistic influences which 

 the host may exert on the transplant may make it necessary for the transplant 

 to have a minimum optimal size before it is able to restitute the whole. We 

 may again refer in this connection to the experiments of Issayew, which 

 have shown that particles from different individuals may be joined together 

 in such a way that they form a whole organism, which represents the mosaic 

 of a chimaera. In this case each particle forms part of one whole, at the 

 same time still retaining its own organismal differential. 



In hydrozoa, as has been noted, the tendency exists to produce single 

 individuals through processes of coalescence, as well as of splitting leading 

 to supplementary newformations. Correspondingly, stolons of hydroids 

 belonging to the same colony, and even stolons from adjoining colonies, 

 may coalesce. Also, larvae of coelenterates may unite among themselves and 

 give origin to a new colony, or they may join a part of an already existing 

 colony and help to enlarge it. However, the mechanism which usually leads 

 to colony formation is that of budding. The question may be raised whether 

 it is the individual polyp or the colony of polyps which shall be considered 

 as the bearer of the individuality. As far as the individuality differentials 

 are concerned, we may assume that, provided they exist at all, they are the 

 same in all component parts of a colony. Even strange colonies belonging 

 to the same species may have identical individuality differentials if they 

 have developed from buds given off by the same colony. But differences in 

 conditions analogous to individuality differentials in such colonies might pos- 

 sibly have originated under the influence of different environmental factors, 

 which were able to modify the different colonies ; furthermore, it is con- 

 ceivable that somatic mutations might occur and lead to such changes. In 

 higher individuals, differences in individuality differentials are, as a rule, 

 due to processes which take place during the formation of the germ cells 

 and during fertilization. We may therefore expect less sharp differences in 

 the nature of the individuality differentials in organisms propagating by 



