468 C. M. Child. 



other cases also. The formative effect of these conditions may 

 conceivably be twofold; they may act as stimuli to growth or other 

 changes, i. e., they may exert a "trophic" effect as Triepel and 

 others have pointed out, or they may act in a direct mechanical 

 manner, bringing about a particular arrangement of material. 

 Both of these methods of action are important but the second 

 has been much neglected in the analysis of formative conditions. 



The direct mechanical effect of pressure and tension upon the 

 form of parts is, I believe, of great importance and may afford in 

 some cases a simple explanation of phenomena which appear 

 inexplicable from other points of view. A good case in point is 

 the change of form called by Morgan "morphallaxis" in regu- 

 lating pieces of Planaria and other Turbellaria. In the case of 

 Stenostonia I have shown this change to be primarily mechanical 

 in nature (Child, '02, '03a) and there is no doubt that in other 

 forms the same factors are effective. In the case of Leptoplana 

 the effect of mechanical conditions has already been shown in the 

 preceding paper (Child, '04), and will be further considered in the 

 present paper. 



But in many cases an indirect relation exists between the nerv- 

 ous system and the mechanical conditions, as in the cases of Sten- 

 ostoma and Leptoplana above mentioned, since the mechanical 

 conditions effective here depend upon the use of the parts in a 

 characteristic manner during locomotion. It is thus easy to see 

 how factors, simple in themselves and entirely independent of the 

 nervous system, may apparently stand in relation to it. The same 

 is of course true with regard to other functional conditions as well 

 as the mechanical factors. 



Even in cases where a direct relation between the nervous system 

 and form may be shown to exist I see no necessity for assuming 

 the existence of special "formative stimuli" or "trophic stimuli" 

 as distinct from the functional stimuli. Moreover, extreme 

 caution is necessary before concluding that a direct relation exists. 



The problem of organic form is undoubtedly the most complex 

 and difficult of all biological problems. I do not think that the 

 suggestions made here tend toward its simplification. The factors 

 of organic form include all the activities of organic substance as 



