Stiulu's on Regulation 359 



exists, of course, for each region, but the conditions in the terminal 

 regions, for example, must he in general typically different from 

 those in the middle region. 



To sum up: the turbellarian intestine as an organ of digestion 

 and a store-house of reserve material is undoubtedly the seat of 

 typical chemical reaction-complexes. As a reservoir for the tem- 

 porary storage of undigested food and as a vascular system con- 

 taining moving fluid it is undoubtedly subjected to a typical com- 

 plex of mechanical conditions. These two groups comprise, I 

 believe, the most important functional conditions for the turbel- 

 larian intestme. 



The intestine of higher forms, or at least some part of it, serves 

 as a place of temporary storage for undigested food and often, as 

 in certain birds and mammals, undergoes a high degree of special- 

 ization in connection with this function. But in higher forms 

 where a specialized circulatory system is present, the intestine 

 does not function to any great extent as a system for the distri- 

 bution of nutritive material and is not subjected to the mechanical 

 conditions which must exist in such a system, although of course 

 mechanical functional conditions are more or less important fac- 

 tors in the functional complex in all cases. There can be no 

 doubt, however, that mechanical conditions constitute a much 

 larger element in the functional complex characteristic of the 

 turbellarian intestine than they do in higher forms. If functional 

 factors play any part in development and regulation, we may 

 expect to find the determining factors in the two cases different to 

 a greater or less extent. 



2 Functional Factors in Intestinal Development and Regulation 



It is a well-established fact that the mechanical conditions con- 

 nected with the movements and pressure of fluid within the vessels 

 are factors of great importance in determining diameter, distri- 

 bution, angle of branching and character of the wall of the blood- 

 vessels. Since this is the case it is natural to expect that similar 

 conditions will play a role of greater or less importance in develop- 

 ment and regulation of the turbellarian intestine. 



