Rfgciwrnfioit (IS Functional Adjiistiuoit 423 



tiition of a larger or smaller part of the old tissues of the piece 

 for the part removed; the substitution may be imperfect or incom- 

 plete at first, and gradually attain completeness. In consecjuence 

 of this functional substitution, the structure of the part involved 

 is altered until it comes to resemble more or less closely that of 

 the part removed." 



Where regeneration through the formation and difi'erentiation 

 of new tissue occurs, it is this tissue which becomes the func- 

 tional representative of the old part. In the regeneration, for 

 instance, of the arm of a star-fish, or the leg of an arthropod or 

 amphibian, the new tissue "must be subjected to many condi- 

 tions — internal and sometimes external — similar in a greater or 

 less degree to those to w^hich the part removed or some portion ot 

 it was subjected." 



The factor of the exercise of a part which Child has regarded in 

 many cases of so much importance, is not always necessary for 

 regeneration. "The growth of the new leg is not the result of 

 the attempt to use the leg which is missing. The growing tissue 

 begins to develop into a leg because its relations to the other parts 

 of the system are in some degree similar to those of the leg 

 removed. As it grows, the conditions approach much more and 

 more nearly those to which the normal leg is subjected, i.e., there 

 is a gradual return of the functional conditions to the normal." 



The application of Child's theory to the subject of polarity, 

 heteromorphosis, and many other regulatory phenomena, it will 

 not be necessary for our present purpose, to discuss. The funda- 

 mental idea of the theory is that form regulation is a result of func- 

 tional regulation. 



In certain cases Child attempts to show that this supposed 

 functional relation actually occurs as a consequence of mechanical 

 conditions. The anterior end of a planarian gets stimulated by 

 the water and by the impact from foreign bodies much like the 

 head does. The posterior end of Stichostemma is used by the 

 animal in locomotion much as the tail is. At first the external 

 stimuli aflfecting the ends are much the same whether they are 

 anterior or posterior. In the movements of the animal both re- 

 ceive frequent impact from contact with various objects. There 



