552 C. M. Child. 



part appears long before the characteristic form is established. 

 In Leptoplana, for example, when the head is cut off just anterior 

 to the ganglia, or in Planaria at any level, the new tissue growing 

 out from the cut surface begins to function in the manner charac- 

 teristic of the head long before it has acquired the form of a head. 

 All the facts indicate that the definitive form is the result of this 

 activity. Here the indirect influence of the nervous system is a 

 necessary factor in the result. Only the earliest outgrowth of 

 tissue before motor activity appears is probably due in large part 

 or wholly to the stimulus of the wound and altered local conditions. 



If we attempt to interpret the close parallelism between motor 

 activity and regeneration in Leptoplana in accordance with the 

 hypothesis proposed above we may suppose that the motor activity 

 exercises a formative eff^ect upon the regenerating part and so 

 determines its fate. But in order that coordinated motor activity 

 may occur in a given part some degree of differentiation into con- 

 tractile and conducting structures must exist. We may regard 

 the regeneration of particular parts of the nervous system in the 

 new tissue as dependent primarily upon functional stimuli pro- 

 ceeding from the old, already organized part of the nervous system. 

 In this relation we have a case where the nervous stimulus and the 

 functional stimulus are identical. 



The muscles in the new part may develop under the influence, 

 direct or indirect, of stimuli proceeding from the nervous system, 

 though it must be borne in mind that visible morphological con- 

 nection between these parts and the nervous system may not be 

 present. In certain other cases, however, as in that of the legs of 

 Triton described by Wolff ('02) and discussed by Goldstein, the 

 definitive form is apparently independent of the characteristic 

 motor activity, though in these cases the physical conditions 

 resulting from differences in the rapidity of growth of different 

 parts may play a role. The hypothesis of the physiological 

 formative influence of function is undoubtedly important but 

 still another possibility remains to be considered, viz: the direct 

 mechanical effect of mechanical conditions. That the arrange- 

 ment and distribution of material is in many cases the direct 

 effect of mechanical conditions cannot be doubted. 



