hunt: regenerative phenomena in earthworms. 579 



The dorsal pouch appeared only in cases where the nerve cord as 

 well as the digestive tube had been excised from the anterior extremity 

 of the operated worm. It was not found in the sixty-seven cases in 

 which only the digestive tube was removed. In each of the five cases 

 the nerve cord regeflerated dorsally, coming into intimate association 

 with the invagination. These facts strongly suggest that the develop- 

 ing invagination was the source of a stimulus which acted upon the 

 cord and determined its direction of growth, and also that the cord 

 may have exerted a similar influence upon the invagination. 



These pouches may be abnormally located stomodea, but at present 

 all we can say is that there seems to be some reciprocal formative 

 relationship between them and the regenerating nerve cord. 



The experiments described in the first group show that the anatom- 

 ical units of the earthworm's body exhibit a striking degree of inde- 

 pendence of each other in anterior regeneration. 



But the action of hormones was not excluded by the methods of 

 experimentation. Possibly the regenerating alimentary canal pro- 

 duced substances which were carried forward to the anterior end and 

 there played a necessary part in the regeneration of head structures. 

 Likewise, these developing head structures may have affected the 

 transected end of the digestive tube in a similar way. It would be 

 hard to determine the role of hormones in these experiments, for even 

 if one could experimentally produce a living animal deprived of its 

 alimentary canal (I was unsuccessful in this attempt), and then could 

 observe the regeneration of the head in such a worm, there would still 

 be the possibility of hormone action, for hormones produced by the 

 alimentary canal during the normal life of the worm might persist 

 in the remaining tissues. 



However, the experiments did eliminate several possible mechanical 

 factors. The anterior region of the nerve cord and digestive tube 

 can be reformed when the transected ends of these organs are several 

 segments away from the front end of the body. Consequently 

 forward growth of the tube and nerve cord does not depend upon 

 mechanical stimulation of these organs by the regenerating parts at 

 the anterior end of the body. 



Conversely, the replacement of a head is not contingent upon 

 mechanical stimulation or support furnished the regenerating cephalic 

 organs by the alimentary canal, as the following facts show. (1) 

 The brain and connectives do not need the presence of the digestive 

 tube as a sort of supporting scaffolding around which to rebuild 

 themselves. (2) The bifurcation of the regenerating nerve cord to 



