318 C. M. CHILD 



SUMMARY 



1. In Planaria dorotocephala (also in P. maculata) two zooids 

 are commonly present during the asexual period, a longer ante- 

 rior and a shorter posterior. The presence of these zooids is 

 indicated by certain regional differences in the character, method 

 and rapidity of regulation of isolated pieces. 



2. In the anterior zooid the character, the method and the 

 rapidity of regulation are dependent upon the length of the piece 

 and upon the region of the body from which it is taken. 



3. The constitutional capacity for head formation decreases 

 from the anterior regions of the first zooid posteriorly the capacity 

 for tail formation from posterior regions anteriorly, but correla- 

 tive factors may determine that heads or tails arise at levels of 

 the body where the constitutional capacity is slight. 



4. The capacity to produce a 'whole' decreases with decreas- 

 ing length of the piece, until in pieces below a certain length, 

 which differs in different regions of the body, partial structures 

 instead of wholes arise. There is no sharp limit between 'wholes' 

 and partial structures {e.g., tailless heads or headless tails.) 



5. The size limit in the formation of wholes and the appearance 

 of partial structures in pieces below this size limit indicate that 

 polarity consists essentially in a dynamic gradient or gradients 

 along an axis, together with the correlative factors resulting from 

 such a gradient. According to this hypothesis, wholes arise only 

 from pieces which include a certain fraction of the gradient or 

 gradients. 



6. That the dynamic gradient is not uniform is indicated by 

 the fact that longer pieces are necessary for the formation of 

 wholes and even for continued existence in some regions of the 

 body than in others. 



7. 'Potence' is not necessarily inherent in the part involved. 

 The head-forming potence of certain pieces, for example is shown 

 to depend, not merely upon the parts directly involved in the 

 development of a head, but upon the piece as a whole. We can 

 even determine whether a head siiall or shall not form at the 



