STRUCTURES RESEMBLING ORGANIC GROWTHS. 159 



substances have been most generally employed (Sachs and 

 others). 1 



Among the various phenomena of organic growth and develop- 

 ment, regenerative processes seem most evidently open to inter- 

 pretation in the light of the foregoing experiments. In the 

 regenerating organism growth or development is initiated as a 

 result of the removal of one of its parts, the usual result being 

 the restoration of the missing part or its equivalent. In many 

 cases the part removed is replaced by growth at the cut surface; 

 in other cases the remaining portion of the organism undergoes 

 a more or less extensive reorganization, called by Morgan 

 "morphallaxis"; in still others, especially in plants, removal of 

 an organ initiates the development of a similar organ from 

 another locality, e. g., when the removal of the growing tip 

 induces the development of lateral branches from axillae or other 

 regions at a distance. 



The first class of cases, where growth at the cut surface restores 

 the missing part, may be compared with the case of a membrane- 

 forming metal in a ferricyanide solution, e. g., a piece of iron wire 

 in which the filamentous or membranous growth has accumulated 

 to a degree sufficient to insulate the metallic surface from the 

 surrounding solution, i. e., to arrest the flow of current between 

 anodic and cathodic regions by the interposition of the non- 

 conducting coating of precipitate. If then a cut be made exposing 

 a fresh surface of metal, the formation of precipitate is at once 

 resumed and continues until a new state of equilibrium is estab- 

 lished; the accumulation of electrolytically formed structure 

 will then be approximately the same as before. The whole 

 structural system, consisting of the metal with its outer coating 

 of precipitate, will exhibit essentially the same constitution 

 as before. Again in the case of a strip of zinc encircled by a 

 copper wire, as in the experiments described above, the zinc 

 forms the anode of the couple, and its surface is soon covered by 

 precipitation-structures, whose accumulation progressively re- 

 tards the flow of current and with it the structure-forming elec- 

 trolysis. When the fresh surface of zinc is exposed, the formation 

 of a new filamentous coating follows simply from the reestablish- 



1 For an experimental examination and critique of this type of hypothesis, 

 cf. the paper of McCallum above cited. 



