554 C. M. Child. 



than myself to admit that mechanical factors are not the only fac- 

 tors in these cases, though their importance in determining the 

 result is evident. 



In analysis it is necessary, though in practice often difficult, to 

 distinguish sharply between mechanical conditions as functional 

 stimuli producing a "trophic" or formative effect and mechanical 

 conditions as direct mechanical factors determining the space 

 relations of masses. In the cases cited there is no doubt that 

 tension and growth are connected in one way or another. Even 

 in regard to this point, however, there is room for the question as 

 to whether the effect is mechanical or not. 



It is probable that in many cases the effect of mechanical con- 

 ditions as stimuli is widely different from the mechanical effect. 

 The reaction to the stimulus may be an increased power of 

 resistance to the particular mechanical conditions involved, as in 

 the case of tendon, bone and various other structures of animals 

 and plants. The direct mechanical effect consists in the arrange- 

 ment of material in accordance with the mechanical conditions of 

 tension and pressure. In Leptoplana not only the form and the 

 direction of growth of the regenerated posterior end but the 

 amount of regeneration and the direction of growth seem to de- 

 pend to a considerable extent upon, and to correspond in direction 

 with the mechanical tension to which the part is subjected (Child, 

 '04a, '04b). It is difficult to determine how far this result is 

 directly mechanical and how far it is due to mechanical stimuli 

 to growth. I think, however, that the presence of both factors 

 must be admitted. But in most cases the elongation of the new 

 tissue in the direction of tension does not continue indefinitely; 

 it gradually decreases and after a time ceases even though the 

 mechanical conditions continue as before. Undoubtedly in many 

 cases the differentiation of the regenerated tissues and their in- 

 creased power of resistance to mechanical tension determine when 

 the elongation shall cease. This may be regarded as somewhat 

 similar to the effect of mechanical conditions on tendon, bone, 

 etc. But in some cases it is possible that the definitive form is 

 merely the condition of mechanical equilibrium. In either case 

 the part retains approximately the same form, provided that the 



