PROCESS OF REGENERATION 



115 



segments, a fission plane in the form of a constriction appears 

 posterior to the middle of the body. This constriction is coinci- 

 dent with a septum ; the anterior end of the new zooid arises be- 

 tween this septum and the succeeding bundles of setae, and the 

 region between the septum and the preceding bundles of setae 

 develops a new posterior end for the old animal. All structures 

 develop completely before separation of the zooids takes place. 

 Usually but two zooids are present. In Dero, the region where 

 the fission plane is to appear is not detectable by an increased 



Fig. 9 Graph of the axial gradient of Dero limosa, showing the secondary 

 posterior rise typical of oligochaetes. 



susceptibility to cyanide, as is the case in Aeolosoma. In fact, 

 even after the fission plane is visible, there is no increased sus- 

 ceptibility at that point. It is only after differentiation has begun 

 on the two sides of the fission plane that any alteration of suscepti- 

 bility is noticeable. At first this consists in a slight increase in 

 susceptibility at the anterior end of the second zooid — that is, 

 disintegration begins at the anterior end of the first zooid, pro- 

 ceeds back some distance, then attacks the anterior end of the 

 second zooid, then the posterior end, and finally the remaining 



