SfucJirs 0)1 Rcffidafion 



395 



cal conditions, and can be interpreted only on the basis of this 

 correlation. There can be little doubt that intestinal regulation 

 in various other species of turbellaria will prove to be similarly 

 dependent on mechanical conditions. 



The history of the pieces without pharynges shows how little 

 significance there is in description or discussion of "reversal of 

 development" without consideration of the dynamic factors in- 

 volved. When these dynamic factors act in reverse sequence and 

 direction from that typical of normal development, then, and not 

 otherwise, does reversal of development occur. There is no law, 

 such as certain authors seem to postulate, that causes an organism 

 to return more or less completely to an earlier stage of develop- 

 ment if deprived of food, or under other changed conditions. 

 The so-called "return" usually consists simply of the loss of previ- 

 ous differentiation, but this does not necessarily constitute a rever- 

 sal, for the method of loss may be very different from the method of 

 acquirement, as in the present case. Moreover, the loss of the 

 original structure or differentiation may be merely the first step 

 in the development of something new in response to altered con- 

 ditions, as is the case in the pieces without pharynges. These 

 pieces are in no sense returning to an earlier stage of development 

 or "embryonic condition," because they lose the old intestinal 

 branches, but are merely undergoing a process of functional adap- 

 tation or regulation. The gradual simplification in intestinal 

 structure, which occurs in various planarians in the course of 

 starvation and reduction in size, is undoubtedly essentially a func- 

 tional regulation just as truly as is the appearance of new branches 

 under other conditions. 



Objection to mv interpretation of the facts may perhaps be made 

 on the ground that the recent experiments of Babak ('06) with 

 amphibia indicate that chemical factors are much more important 

 than mechanical in determining intestinal regulation. It can 

 scarcely be doubted, however, that the amphibian intestine differs 

 greatly from the turbellarian in function. As I have pointed out 

 in Section I, the turbellarian intestine is much more than a digestive 

 organ, being both a storage-reservoir for excess of undigested food- 

 material and to a considerable extent a circulatory system. It 



