DYNAMICS OF MORPHOGENESIS 439 



In the first paper of this series (Child '11 c) it was shown that 

 the frequency of head-formation decreases as the length of the 

 piece decreases and also from anterior to posterior regions. In the 

 present paper we have seen that the degree of stimulation increases 

 as the length of the piece decreases and also that the degree of 

 stimulation is greater in posterior than in anterior pieces. It 

 would seem that these two features, degree of stimulation after 

 section and frequency of head formation, must be in some way 

 associated or have a common foundation. But before the re- 

 lation between them can be made clear it is necessary to find 

 when in the history of a piece it is determined whether the piece 

 shall give rise to a head or not. And consideration of this prob- 

 lem, the time of head determination, must be postponed to an- 

 other paper. The chief purpose of the present paper, however, 

 is to point out this inverse relation between head frequency and 

 degree of stimulation after section. The following paper will 

 afford an insight into the nature of the relation. 



5. After the ternporary increase in rate of metabolism following 

 section has disappeared a second slow and (relatively) permanent 

 increase in rate occurs, in connection with reconstitution and this is 

 a process of rejuvenescence resulting from the reorganization and 

 reduction of the pieces. 



The writer has called attention elsewhere (Child '11 a, '13 d) 

 to the occurrence of rejuvenescence in connection with experi- 

 mental and asexual reproduction in planarians. The suscepti- 

 bility tables in the present paper show merely the earUest stages 

 of this process at one hundred and twenty hours after section 

 (tables 4, 8, 12). This slow increase in rate in the pieces, which 

 begins after three or four days and which indicates the beginning 

 of rejuvenescence is relatively permanent, i. e., it disappears only 

 gradually as the animal undergoes growth and becomes older in 

 consequence of feeding. The degree of rejuvenescence is in 

 general inversely proportional to the size of the piece and di- 

 rectly proportional to the degree of reconstitutional change. In 

 this connection another expression of the axial gradient in rate 

 of metabohsm may be mentioned, although it is not clearly shown 

 in the tables. In general, the degree of rejuvenescence in recon- 



