104 HARRIET L. ROBBINS AND C. M. CHILD. 



least in the ectoderm and body wall, from the head to the level 

 of fission and rises suddenly at the anterior end of the second 

 zooid and of each further zooid, if such are present. 



It has also been shown that in isolated pieces of the body of 

 Planaria, particularly in those below a certain fraction of the 

 body length, susceptibility is increased during the first few hours 

 following section. This temporary increase of susceptibility, 

 apparently associated with the stimulation of the pieces by 

 section, is least in pieces nearest the head, and increases in suc- 

 cessive pieces posteriorly to the level of fission, where it again 

 decreases (Child, '14). In general the regions of highest sus- 

 ceptibility in the intact animal show the least increase in sus- 

 ceptibility in isolated pieces, and vice versa. Within twelve to 

 twenty-four hours after section the temporary increase in sus- 

 ceptibility has disappeared and the susceptibility of each piece 

 is about the same as, or slightly lower than before section. 

 From twenty-four to forty-eight hours after section a gradual 

 rise in susceptibility begins as the regulatory processes leading 

 to the development of a new individual make themselves evident, 

 and the susceptibility after regeneration is completed is much 

 higher than that of the region of the body of the original animal 

 which a given piece represents. This is true not only for the 

 newly developed head and posterior end of the regenerated animal 

 but for the whole body (Child, '15, Chap. V). In the light of 

 various facts, these changes in susceptibility have been inter- 

 preted as indicating and in some way associated with changes 

 in rate of oxidation. The present paper constitutes additional 

 evidence for this conclusion. Carbon dioxide production was 

 chosen instead of oxygen consumption as the subject of investi- 

 gation first because investigation of the temporary changes, 

 immediately following section involves the technical difficulty 

 of preparing very large numbers of pieces within a very short 

 time in order that oxygen consumption may be sufficient for 

 determination in short periods, and second because work on 

 oxygen consumption during the course of regeneration was 

 already in progress in this laboratory. The colorimetric method 

 was used since it is well adapted for obtaining comparative data 

 with small amounts of material. 



