PROCESS OF REGENERATION 129 



The posterior end is incapable of regenerating an anterior end 

 unless it be of a certain minimal length. The reason for this is 

 again to be sought in the metabolic relations between the piece 

 and the new cells at its cut surface. The posterior end of oligo- 

 chaetes, as I have shown, has a high rate of metabolism; unless 

 the new cells at the cut surface can attain a rate of metabolism 

 sufficiently high to enable them to grow at the expense of the 

 old piece, a head cannot form. 



Head formation exhibits a progressive decrease along the antero- 

 posterior axis. Posterior regeneration also decreases along the 

 axis; anterior pieces produce more segments posteriorly than do 

 posterior pieces. As the head itself does not regenerate readily 

 posteriorly, as already explained, it is the region behind the head 

 which produces the greatest number of posterior segments. 

 The reason for this appears to be that the more anterior the piece, 

 the higher its level in the primary gradient, and the greater the 

 vigor and intensity of its metabolic processes. As the posterior 

 end develops wholly in correlation with the piece, it follows that 

 the more vigorous the piece, the greater the amount of posterior 

 regeneration, i.e., the more nearly does it approach the normal. 

 Morgulis ('07) has given fully the data regarding regeneration 

 of the posterior end in Lumbriculus limosus, and as my own 

 observations regarding number of segments regenerated poste- 

 riorly at the different levels of the body agree with his account, 

 I shall not discuss this point any further. On the contrary, 

 the differences in head formation along the axis are of great im- 

 portance, and will form the subject matter of most of the re- 

 mainder of this paper. 



With these remarks on the general features of regeneration, 

 common to all oligochaetes, I shall now take up the details of 

 regeneration in the four species with which I have worked, — 

 Dero limosa, Lumbriculus inconstans, Tubifex tubifex, and 

 Limnodrilus claperedianus. 



