INTERNAL FACTORS OF REGENERATION IN ANIMALS 



53 



B 



B 



worm are cut off, as shown in the left-hand figure in Fig. 26, and 

 then a piece of the mid-ventral body wall of the worm is cut out, 

 a part of the ventral nerve-cord being removed with the piece. The 

 cut-edges meet along the mid-ventral line and fuse, closing the 

 wound. As a result of the operation there is left exposed, at 

 the anterior end of the worm, a cut-surface with all of the internal 

 organs present except the nervous system. The anterior end heals 

 over, but I have not observed the development of a new head at this 

 level, although the exposed end is in a region at which, under 

 ordinary circumstances, a new head readily regenerates. In several 

 cases a new head developed at the 

 point where the cut-end of the ner- 

 vous system is situated, i.e. at the 

 level B in the figure. 



A variation of the same experi- 

 ment shows still more conclusively 

 the importance of the nervous sys- 

 tem for the result. A few anterior 

 segments are cut from the anterior 

 end as before. A cut is made, as 

 shown in the right-hand figure in 

 Fig. 26, to one side of the mid-ventral 

 line (indicated by the black line in 

 the figure at the level A). Then, at 

 the posterior end of this cut a piece 

 is removed from the mid-ventral 

 line as in the former experiment 

 (shown by the stippled area in the 

 figure). A portion of the ventral 

 nerve-cord is removed with the piece. 

 As a result of this operation, two 

 anterior ends of the nervous system 

 are left exposed (shown by the black 

 dots in the figure). At the anterior 

 end of the worm, i.e. at A, there is 

 one exposure, and at the posterior 

 end of the region from which the 

 piece was removed there is another. 

 Two heads develop in successful 

 cases, one at the anterior end of the anterior cut-surface, i.e. at A, 

 and the other at B. 



The results show that in the absence of the cut-end of the nervous 

 system at an exposed surface a new head does not develop ; and con- 

 versely, the development of a new head takes place when the anterior 



X Y 



FIG. 26. Left-hand figure X shows how, 

 alter cutting oft" the anterior endofA/Mo- 

 bophora fcetida, a piece of the ventral 

 wall (including a part of the nerve-cord) 

 is cut out. Right-hand figure Y illus- 

 trates a more complicated operation, in 

 which the piece of the ventral wall that 

 is cut out is a little behind the anterior 

 end. 



