390 F. H. Krecker, 



in part due to the important fact that the bluntness of the anterior 

 end prevents the worin from cleaving its way into the mud. 



Minimal Size. 



Although the minimal size of a piece capable of regenerating a 

 complete worm has been determined for several annelids still the mini- 

 mal size has not been established for all of the levels within the region 

 capable of head formation. The following experiments deal with 

 levels from the first to the eighth somites and with pieces consisting 

 of from 3 to 7 somites. 



Ist Series — Dec. llth 08. Pieces three somites long were cut 

 at the levels of each of the first eight somites as follows: 1 — 3, 2 — 4, 

 3 — 5, 6 — 8, 7 — 9, 8 — 10. Twenty worms were used for each level. 

 By Jan. 2nd 09 all had died without regenerating except one worm 

 each in the levels 5 — 7 and 6 — 8. The former had regenerated a head ^ 

 and posteriorly two setigerous somites. The piece from the latter 

 level had formed an anal opening but had no head. 



2nd Series — Nov. lOth 08. Pieces four somites long were cut 

 thus; 1—4, 2—5, 3—6, 4—7, 5—8, 6—9, 7—10, 8—11. Twenty in- 

 dividuals were used for each level. All pieces in front of somites 4 — 7 

 died within a week. By Nov. 24th six of the 5 — 8 pieces had regenerated 

 one-fourth segment of tissue posteriorly in which setae were present; 

 one of the 6 — 9 pieces had formed new tissue at the posterior end one 

 Segment long in which were indicated five new somites; one of the 

 7 — 10 pieces had new tissue three Segments long in which were indicated 

 seven new somites and three others had one-half segment of new tissue. 

 Another had one segment of new tissue posteriorly but no head. Both 

 5 — 7 and 6 — 9 had one-fourth segment of new tissue at the anterior 

 end and 7 — 10 had one-fourth segment. A week later one of the 

 4 — 7 pieces had one-half segment of new tissue posteriorly in which 

 one new somite was indicated and another had one fourth segment of 

 new tissue; anteriorly there was one-half segment of tissue. 8 — 11 died 

 in two weeks without regenerating. 



3rd Series — Dec. Ist 08. Ten pieces five somites long were used 

 here, from each level thus; 1—5, 2—6, 3—7, 4—8, 5—9, 6—10, 7—11, 

 8 — 12. A week later aU of 1 — 5 and 4 — 8 had died. Three of the 

 3 — 7 pieces had one-half segment of new tissue both anteriorly and 

 posteriorly. A mouth had been formed and at the posterior end there 



1 Head as used here includes only the prostomium and the peristomium. 



