Studies on Regulation. 555 



mechanical conditions do not undergo marked change. If any 

 considerable permanent change in the conditions does occur the 

 form is gradually altered until a new equilibrium of one kind or 

 another is established. 



To sum up: the close parallelism between the amount and 

 rapidity of regeneration and the form of the regenerated part in 

 Leptoplana and the characteristic motor activity of the part may 

 be accounted for in three different ways: nervous stimuli them- 

 selves may be regarded as formative factors; the functional con- 

 ditions including mechanical conditions in particular parts which 

 are determined by functional relation between the new and old 

 parts may be regarded as stimuli to growth and therefore as 

 formative factors; and lastly, mechanical conditions connected 

 with the characteristic motor activity and other functional con- 

 ditions may determine directly in greater or less degree the 

 arrangement of material and so the form. 



As has been indicated in the preceding paragraphs I believe 

 that all of these factors are concerned to a greater or less extent. 

 The facts regarding regeneration of the ganglia indicate that con- 

 ditions obtaining in the parts of the nervous system from which 

 regeneration occurs are the essential formative factors for the 

 ganglia. Some suggestions regarding the probable role of and 

 mechanical and non-mechanical effects of non-nervous functional 

 conditions in determining form have been made in the preceding 

 paragraphs. My successful attempts at experimental control of 

 form-regulation (Child, '02, '03, '04a) have convinced me that 

 mechanical conditions are of great importance, at least in the 

 cases considered. While I think it probable that they will be 

 found to be equally important in many other cases, though per- 

 haps in widely different manner, generalization would be prema- 

 ture. What has been said in the present section is sufficient, I 

 trust, to make it clear that I do not exaggerate the importance of 

 mechanical factors in morphogenesis. 



The mechanical aspects of morphogenesis have been much 

 neglected; application of mechanical principles will undoubtedly 

 assist us in future in the consideration of organic form. But no 

 single principle of interpretation will lead us far in biological 



