GENERAL INTRODUCTION 9 



This is illustrated in the regeneration of the anterior end of the earth- 

 worm ; for example, in the red-banded earthworm, or brandling (Allo- 

 lobopliora fcetida}. 1 If one segment of the anterior end is cut off, one 

 segment is very quickly regenerated (Fig. 2, B); if two segments are cut 

 off, two come back (Fig. 2, C) ; if three segments are cut off, as many 

 are regenerated (Fig. 2, D) ; if four are cut off, generally four come 

 back (Fig. 2, E); when five are cut off, four or five come back (Fig. 

 2, / r ) ; but if six or more are cut off, only four or five are regenerated 

 (Fig. 2, G). It is found in this case that a limit is soon reached beyond 

 which fewer segments are produced than have been removed. The 

 new segments form the anterior end or head that enlarges to the char- 

 acteristic size ; but the missing segments behind the new head are 

 never regenerated, and the worm remains shortened throughout the 

 rest of its life. If the reproductive region has been removed with 

 the anterior part, new reproductive organs are never formed and 

 the worm remains incapable of reproducing itself. 



This same relation between the number of segments cut off 

 from the anterior end and the number that is regenerated seems to hold 

 good throughout the whole group of annelids, although the maximum 

 number that comes back may be different in different species. Thus 

 in lumbriculus six or seven or even eight new segments come back if 

 more than that number have been removed. 



If we examine the method of regeneration from the posterior end 

 of a piece of an earthworm, we find that when several or many 

 posterior segments have been removed a new part comes back, com- 

 posed at first of a very few segments. The terminal segment 

 contains the new posterior opening of the digestive tract. New 

 segments are now formed just in front of the terminal segment, the 

 youngest being the one next to the end-segment. The process con- 

 tinues until the full complement of segments is made up (Fig. 3, 

 C, D, E}. Comparing these results with those described above for 

 the anterior end, we find, in both cases, that only a few segments 

 are at first formed, but in the posterior regeneration new segments 

 are intercalated near the posterior end. This process of interca- 

 lation is the characteristic way in which many annelids add new seg- 

 ments to the posterior end, as they grow larger and longer. 



Amongst the flatworms the fresh-water planarians show remark- 

 able powers of regeneration. If the anterior end is cut off at any 

 level, a new head is produced (Fig. 4, C}. The new worm is at first 

 too short, i.e. the new head is too near the pharynx, but changes 

 take place in the region behind the new head that lead to the devel- 

 opment of new material in this part. The new head is, in conse- 



1 These experiments on the earthworm are in the main taken from my own results ('95) 

 ('97) ('99). 



