NOTES ON REGENERATION. 163 



posterior end. The middle pieces included the next portion of 

 the worm, and extended to the region of the reproductive pore. 

 These pieces should be capable of regenerating a head at their 

 anterior ends and a tail at the posterior ends. The third pieces 

 were the tail pieces and included the rest of the worm. These 

 pieces should be incapable of regenerating a head at the anterior 

 end. The pieces were preserved at intervals of I, 2, 3, 4, 5, 6 

 days, killed, embedded, stained and examined with immersion 

 lenses. 



A study 9f the sections shows that the changes taking place at 

 the anterior end of the tail -pieces appear to be similar in all 

 respects to those that occur at anterior or posterior surfaces at 

 which regeneration of the missing part takes place. There is 

 nothing in the sections to show why the regeneration should con- 

 tinue in the one case and not in the other, and it is difficult to 

 believe from the evidence of the sections that anterior regenera- 

 tion from the tail-pieces would not in time be accomplished, yet 

 after three weeks there was no sign of further regeneration and I 

 am forced to conclude with Lillie that in the form of Dendro- 

 ccehtin found at Falmouth regeneration does not, ordinarily at 

 least, occur behind the level of the pharynx. Sections through 

 tail-pieces, cut off behind the pharynx and kept for nearly three 

 weeks, show that the formation of new tissue has not gone much 

 beyond that of the first six days, and that a new head has not 

 been produced. Sections of the oblique, and of the pointed tail- 

 pieces give exactly the same results. 



Several writers seem inclined to account for the lack of regen- 

 eration in certain planarians, and especially from the posterior 

 region of the body, as due to the absence or small size of the 

 nerve cords in these regions. With this view I do not agree. 

 Lillie has used Dendrocoeluni as a case in point. Sections of this 

 worm show, however, that the cords in the more posterior regions 

 are as well developed, judging from their size, as they are in 

 Planaria maculata. 



REGENERATION IN PYCNOGONIDS. 



In 1895 Loeb published some observations that he had made 

 on the regeneration of one of the Pycnogonida, PJioxichilidium 



