378 C. M. Child 



a ,short anterior portion of the postpharyngeal region becomes 

 functionally a whole postpharyngeal region and the change in 

 mechanical conditions, although not a reversal as in prepharyngeal 

 pieces, is without doubt great. The fact that when half or more 

 of the original postpharyngeal region remains no degeneration, 

 or practically none, except the usual slow process of reduction 

 common to all specimens without food occurs, points in the same 

 direction. The larger the part of the postpharyngeal region from 

 which the new whole region is formed, the less the change in 

 functional conditions associated with the functional regulation 

 and the less the degeneration. 



The facts as to rapidity of degeneration also support the func- 

 tional hypothesis. The lateral intestinal branches of the rediffer- 

 entiating region disappear most rapidly in short prepharyngeal 

 pieces, where the new postpharyngeal region is formed from a 

 region not far posterior to the cephalic ganglia. The rapidity of 

 degeneration decreases as the level of the region from which the 

 new postpharyngeal region is formed approaches the old pharynx. 

 In pieces without the cephalic ganglia the rapidity of degeneration 

 is somewhat less than in pieces with the ganglia. In those pieces 

 in which the new postpharyngeal region redifferentiates from a 

 short anterior portion of the old postpharyngeal region the dis- 

 appearance of the intestinal branches is still less rapid than in the 

 longer prepharyngeal pieces and, as noted above, in those cases 

 where half or more of the old postpharyngeal region remains, the 

 branches persist and undergo reduction in the usual manner. 



It is not difficult to understand from the functional standpoint 

 why these differences in rapidity of degeneration should occur. 

 The change in the mechanical conditions in the intestine must be 

 greatest when a region originally just posterior to the cephalic 

 ganglia redifferentiates into a postpharyngeal region and least 

 when the new posterior end is formed from a large part of the old 

 postpharyngeal region. Between these two extremes the change 

 is intermediate in degree. Evidently then the rapidity of degener- 

 ation in these cases is, as might be expected, parallel to the degree 

 of change in the mechanical functional conditions. In the pieces 

 without the ganglia movement is somewhat less energetic and less 



