Studies on Regulation. 545 



The region posterior to the pharynx does not show as great a 

 degree of motor activity as the anterior region and we find corre- 

 spondingly that the development of the intestinal branches is less 

 rapid here than there. 



The growth of intestinal branches in a postero-lateral direction 

 from the pharyngeal region and in an antero-lateral direction 

 from the region posterior to the pharynx is also of much interest. 

 In none of these pieces did lateral intestinal branches regenerate 

 from the pharyngeal region. 



We may regard the branched form of the polyclad intestine as 

 primarily the result of distension by internal pressure of a sac sur- 

 rounded by parenchymatous tissue. The distension occurs along 

 paths of least resistance and greatest internal pressure. Its 

 direction is the result of several factors, viz: the direction of inter- 

 nal pressure, the position of paths of least resistance, and the 

 presence or absence of antagonistic external pressure, however 

 produced. 



Since the internal pressure in each branch is transferred in some 

 degree to the tissues about it this pressure must constitute for an 

 adjoining branch external pressure antagonizing the internal 

 pressure in the second branch and so preventing its extension 

 toward the first. In short, intestinal branches cannot enter a 

 region already occupied by such branches unless the internal 

 pressure in the former is much greater than in the latter. 



In the case under consideration at present, viz: the growth of 

 intestinal branches in "abnormal" directions from the regions 

 anterior and posterior to the pharynx into the pharyngeal regions, 

 I believe that the absence of intestinal branches in this region is 

 the determining condition. The intestinal contents, being forced 

 into the branches anterior and posterior to the pharynx, exert 

 pressure upon the walls of these and growth occurs along the lines 

 of least resistance. Under ordinary conditions the presence of 

 other similar branches would in consequence of the pressure upon 

 the tissues prevent the growth of branches postero-laterally in the 

 one case and antero-laterally in the other (Fig. 61). Here, how- 

 ever, such branches are absent and consequently the mechanical 

 resistance to extension of the branches in this direction is not 



