396 C. M. Child 



would be remarkable if mechanical factors were not much more 

 important functionally in the turbellaria than in the amphibia. 

 In order to mterpret regulatory phenomena it is of the utmost 

 importance to consider all the functions of an organ or structure 

 and not merely one, or the most conspicuous. Only in this way 

 shall we attain complete interpretation. It must be borne in 

 mind that the name assigned to a part does not always indicate 

 fully or exactly its functions, nor is the function commonly assigned 

 to it necessarily its only function: in most cases it is merely a small 

 part of the actual function. 



In the present case the changes in mechanical conditions are to 

 a certain extent visible and accessible to experimental methods and, 

 as I have endeavored to show, the processes of regulation in the 

 intestine are evidently closely correlated with them; indeed it is 

 impossible to account in any other way for certain of the changes, 

 such as the rapid degeneration in a postpharyngeal region formed 

 by redifferentiation and the development of new intestinal branches 

 in pieces without pharynges after months of starvation. More- 

 over, while other factors, such as the character of the food and the 

 digestive activity, doubtless affect the structure of the cells and 

 very probably their number, it is difficult to understand how fac- 

 tors of this kind alone can determine the form, arrangement and 

 direction of intestinal branches. These elements of the intestinal 

 form must, it seems to me, be determined mechanically, at least 

 in large part, and it is with these that the present paper is primarily 

 concerned. 



The most important results are briefly stated in the following 

 summary: 



1 In normal animals kept for several months without food 

 extensive intestinal degeneration occurs, beginning in the peripheral 

 regions and proceeding toward the pharynx. This degeneration 

 involves chiefly the lateral branches and affects the axial intestine 

 only in the terminal regions. 



2 In pieces undergoing regulation without food in which a 

 postpharyngeal region is formed by redifferentiation from a part of 

 the old prepharyngeal or the anterior part of the old postpharyn- 

 geal region, the old lateral branches of the intestine undergo rapid 



