28 PATTERNS AND PROBLEMS OF DEVELOPMENT 



terns occur, but the chains usually consist of fewer zooids because growth 

 in length and physiological isolation of the posterior regions of existing 

 zooids are relatively less rapid than development of zooids. In some poly- 

 chete annelids chains of zooids occur, but the posterior members usually 

 separate successively before they undergo fission. Fissions provide ex- 

 amples of reconstitution occurring under natural conditions, in conse- 

 quence either of physiological isolation or of physical isolation. The polar- 

 ity and ventrodorsality of the new individual are usually in the same di- 

 rection as in the parent : the new head develops at the most anterior re- 

 gion of the part, which becomes the zooid, and its ventrodorsal pattern is 

 apparently determined by the parent ventrodorsality. Except as regards 

 effects of physiological isolation of parts in initiating development, the 

 natural fissions give us little information concerning developmental pat- 

 tern that is not obtainable from the reconstitutions resulting from experi- 

 mental section, and they have the disadvantage for experimental analysis 

 that they occur only under certain physiological conditions, although 

 these conditions can be induced experimentally in some forms, and are 

 usually limited to certain regions of the body. The wider range of possi- 

 bilities of analysis in the reconstitutions following experimental section 

 will become evident in the following section. 



POSTEMBRYONIC RECONSTITUTIONS 



Reconstitutional development may be defined as the alteration of pat- 

 tern and course of development in a part of a pre-existing individual fol- 

 lowing its physical or physiological isolation from its normal organismic 

 environment. In other words, it is the realization, in the isolated part, of 

 other developmental potentialities than those which have been, or would 

 be, more or less fully realized without isolation. This form of develop- 

 ment has been variously called "regeneration," "restitution," "repara- 

 tion," "reproduction," and, in general, "form regulation." For Driesch 

 and many others it has involved the assumption that pattern and de- 

 velopment of the isolated part are so altered that it approaches in some 

 degree or becomes a whole, a normal individual; that is, the isolated part 

 gives rise to more of the individual than if it had remained an integrated 

 part of the original whole. This conception of reconstitution is implied in 

 the terms "restitution," "reparation," "regeneration," and "form regula- 

 tion." While it is, of course, true that very often an isolated part does be- 

 come more nearly a whole, it is also true that very often it does not. Its 

 pattern may be so altered that it gives rise to some other part or parts of 



