PEOCESS OF REGENERATION 159 



5. In zooid formation, the gradient of the zooid gradually 

 becomes independent of the gradient of the parent animal and 

 is of the primary form. Owing to the processes of growth and 

 dediff erentiation involved in zooid formation, the rate of metab- 

 olism of the fully developed zooid is higher than that of the 

 parent; i.e., rejuvenescence results from asexual reproduction. 



6. In oligochaetes a certain number of the most anterior seg- 

 ments are differentiated as a head. 



7. In regeneration, the head and tail are replaced by out- 

 growth, the other parts by reorganization of the old tissue. No 

 matter how many anterior segments are removed, only the typical 

 number of head segments is, in general, replaced. 



8. The head of oligochaetes will not regenerate a tail unless a 

 certain number of trunk segments are included with it; nor will 

 the end of the tail regenerate a head unless of a certain minimum 

 size. Explanations of these facts are suggested. 



9. In Dero limosa, any part of the body, whether long or 

 short, regenerates a normal worm (with exceptions noted in 8). 



10. In Lumbriculus inconstans, any part of the body, if of 

 sufficient length, regenerates a normal worm. Short pieces show 

 progressive inhibition of head formation along the axis; they 

 give rise to anterior structures showing all gradations between 

 a normal head and a normal tail. Normal posterior regenera- 

 tion occurs at any level, and with any size of piece, but the 

 number of segments regenerated decreases along the ■ antero- 

 posterior axis. 



11. In Tubi£e:?, .head formation ceases at about the level of 

 the fifteenth anterior segment, and, in Limnodrilus, at the level 

 of the seventh segment, regardless of size of piece. Tail for- 

 mation occurs at any level. 



12. In Lumbriculus inconstans, it is determined, whether or 

 not the head shall be normal within twenty to twenty-five hours 

 after the pieces are cut. 



13. The gradient of an axial series of pieces is not the same as 

 that of a whole worm, because cutting stimulates. This stimu- 

 lation is greater the shorter the piece and the lower its previous 

 rate of metabolism. This stimulation is temporary, the time of 



