Studies on Rcgidatio)! yj^ 



direction and extension in the posterior direction, whereas the 

 reverse was originally the case. The intestinal contents now tend 

 to enter the more anterior branches of the region during contrac- 

 tion and the more posterior during extension, but the branches 

 were previously subjected to conditions the reverse of these. 

 These altered conditions must bring about a very different distri- 

 bution of the pressures and strains on the various parts of the intes- 

 tine in this region. If the outline, arrangement and direction of 

 the intestinal branches is determined in any marked degree by 

 mechanical factors connected with the presence and movements 

 of fluid contents, it seems impossible to doubt that such an extreme 

 change in these factors must result either in a transformation of 

 the original structures or in their disappearance, for they are the 

 product of conditions the reverse of those now existing. Appar- 

 ently the change is too great to permit transformation and the 

 old structures disappear. 



Moreover, if these mechanical conditions determine the intes- 

 tinal changes in these cases, the persistence of the axial intestine 

 is to be expected, for the functional conditions in it remain essen- 

 tially as before, the direction of movement of the contents being 

 merely reversed in each particular instance. Only slight quanti- 

 tative changes, if any, are to be expected, therefore, in the axial 

 intestine. As a matter of fact, the only change observed in the 

 axial intestine in these pieces is a change in diameter in different 

 regions. Instead of remaining larger as originally in case the 

 piece was prepharyngeal, the posterior part becomes smaller than 

 the anterior, a change which is doubtless correlated with the new 

 functional conditions. 



But the fact that the pieces in which the new postpharyngeal 

 region redifferentiates from a short anterior portion of the old 

 postpharyngeal region show the same rapid disappearance of the 

 intestinal branches may perhaps be regarded as an objection to 

 this hypothesis. It may be said that in these cases the mechanical 

 conditions are not altered in the same manner and degree as in the 

 prepharyngeal pieces and that the intestinal degeneration cannot, 

 therefore, be due to such alteration. This objection cannot hold, 

 however, as a moment's consideration will show. In these cases 



