Studies on Regulation. 467 



upon analysis will be found to be indirect rather than direct. It 

 is to be remembered, however, that even in cases of this kind a 

 direct relation may also exist, /. e., the functional nervous stimuli 

 themselves may conceivably exercise a direct influence of some 

 sort upon the form. 



The question as to whether the complex of functional conditions 

 exclusive of nervous stimuli may effect form is undoubtedly to be 

 answered in the affirmative. The existence of a relation between 

 these conditions and form has been established with more or less 

 certainty for various cases by Roux and others. Little attempt 

 has been made, however, to analyze these conditions or analysis 

 has usually not proved very successful. The best examples of 

 so-called functional structures are to be found in the tissues con- 

 nected with movement. In these structures the arrangement of 

 parts is very closely dependerit upon the conditions resulting from 

 use of the organs in a characteristic manner. 



In most of the Turbellaria as well as in many other forms the 

 whole body is more or less involved in the characteristic move- 

 ments and thus becomes in a sense a complex motor organ. It is 

 not improbable therefore that the various conditions to which the 

 tissues are subjected in consequence of the characteristic move- 

 ments are in certain cases important formative factors. I have 

 already shown that such conditions are concerned in form 

 regulation in Stenostoma and Leptoplana (Child, '02, '03a, '04). 

 As will appear, the description in the following section of the rela- 

 tion between the nervous system and motor activity in Lepto- 

 plana is a necessary preliminary to the experiments to be presented. 



To what extent the functional conditions may constitute forma- 

 tive factors in cases where motor activity is not concerned is a 

 problem regarding which the data are at present few. I am in- 

 clined to believe, however, that we shall find form to be essentially 

 functional in very many cases where it is not at present so re- 

 garded. Indeed in one sense all organic form is functional. 



Among the conditions resulting from functional activity me- 

 chanical conditions are important. Their importance has been 

 recognized in connection with the structure of bone, muscle and 

 connective tissue, but I think they are important factors in many 



