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C. M. Child 



differentiated parts, could persist in a system such as Holmes 

 postulated. If it has no function in the system and is not in 

 symbiotic relation with other parts, why should it not disappear ? 

 If, on the other hand, symbiotic relations between it and other 

 parts exist, it should, according to Holmes, differentiate into some- 

 thing. It is evident therefore that something besides undiffer- 

 entiated tissue must take the place functionally of the part removed 

 if replacement is to occur. 



We can most readily gain an idea of what this something is by 

 means of a concrete example. In Planaria and various other 

 triclads, where even small pieces are capable of replacing all parts, 

 we find that the reactions of such pieces, while differing in degree 

 from those of the original animal, do not differ essentially in kind 

 After removal of the head, for example, the piece reacts in much 

 the same manner as when the head was present, though more 

 slowly and with less energy. In Leptoplana, on the other hand, 

 where regeneration of a head does not occur after removal of the 

 ganglia, the piece without gangha is at once and clearly distinguish- 

 able from the animal with gangha by the character of its reactions. 



In Planaria then, and in the other forms where replacement of 

 the head and gangha are possible, the piece still retains in some 

 degree the functional characteristics of a head-region. In remov- 

 ing the head we have not removed the only region possessing such 

 characteristics, but only the region which possesses them in the 

 highest degree of any part of the animal. In Leptoplana removal 

 of the head and gangha leaves no part which can supply function- 

 ally, even in slight degree, their place, and formation of a new 

 head is impossible. 



We must conclude that the locahzation of visible structural 

 differentiation in an organism is not necessarily coextensive with the 

 locahzation of functional processes or conditions characteristic of 

 this region, but may be limited to the region of greatest energy of 

 these processes or conditions. It is a well-recognized fact that the 

 so-called functional structure of bone, tendon, etc., represents only 

 the most frequent or most energetic functional conditions, and 

 there is every reason to beheve that similar relations exist between 

 structure and function in many other cases. The case of Planaria 



