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C. M. Child 



regulation occur in cases where return to the typical form of the 

 species does not occur. 



In the earher papers (Child '05a, '05b, '05c) dealing with this 

 species the processes of form-regulation were interpreted as essen- 

 tially cases of functional regulation, i.e., "functional adaptation." 

 For example the redifferentiation into a posterior end of the pos- 

 terior part of a prepharyngeal piece and the formation of the 

 pharynx at a certain level of the old tissue was regarded as the 

 result of a functional regulation in response to altered conditions, 

 in consequence of which a portion of the body which had been 

 functionally, as well as morphologically, prepharyngeal now 

 became functionally posterior, i.e., postpharyngeal, and in conse- 

 quence underwent regulation, /. e., functional adaptation of its 

 structures to the new conditions. 



As I have pointed out repeatedly in different papers (Child '05a, 

 '06a, '06b), redifferentiation of old parts into parts similar to 

 those removed can occur only when these old parts are capable in 

 some degree of becoming the functional representatives or substi- 

 tutes of the parts removed. If functional substitution for the 

 part removed does not occur at all, form-regulation does not occur: 

 if the substitution is confined to regions adjoining the cut surface 

 the part is replaced more or less completely by regeneration, the 

 completeness of replacement depending on the degree of functional 

 substitution. 



As was shown in the earlier papers on Cestoplana (Child '05a, 

 '05b, 05c), the phenomena of form-regulation in general can be 

 readily and consistently interpreted on this basis and the differ- 

 ences between anterior and posterior, preganglionic and post- 

 ganglionic regulation, and regulation in the presence and in the 

 absence of the ganglia, differences which on any other basis 

 appear merely as isolated facts without special significance and 

 without relation to each other, are clearly correlated and explicable. 



For the more complete discussion and interpretation of the 

 experimental data in the light of this hypothesis the reader is 

 referred to the earlier papers (Child '05a, '05b, '05c, 06a, '06b). 



Since the phenomena of intestinal regulation are so striking in 

 this species they were omitted from the preceding papers as deserv- 



