354 G - E - GATES. 



one animal died following operation anterior to segment six and 

 in this single instance there was reason for suspecting that the 

 death of the worm was not caused by the effect of the operation 

 on the individual but by another factor to be discussed later. 



When the excision was made at an intersegmental furrow pos- 

 terior to 6/7 but anterior to 18/19 the ratio of the number of seg- 

 ments removed to the number regenerated, varied and the per- 

 centage of failure to survive the operation was higher. Here 

 again there was reason for suspecting that many of the mortalities 

 following operation were due to something more than the effect of 

 the operation itself. None of the amputations at intersegmental 

 furrows 10/11 or 14/15 resulted in hypomeric regeneration, but 

 the number of worms involved (five at the first furrow and two 

 at the second level) is too small to permit generalization. At all 

 other intersegmental furrows between 6/7 and 18/19 hypomeric 

 anterior ends have been formed. The largest number of seg- 

 ments not replaced is four and the smallest is one. 



In one series four worms regenerated hypomeric heads each 

 having one " half segment," i.e., a metamere extending across the 

 whole diameter of the animal but antero-posteriorly only about 

 half the width of the segments immediately in front of and be- 

 hind it. Such half segments were not setigerous and may repre- 

 sent incompletely differentiated metameres, or perhaps growth 

 zones, although the specimens concerned were killed in the fourth 

 week after the operation, by which time the segmental differentia- 

 tion is usually completed. In only one regenerating anterior end 

 was a wedge-shaped half -segment found and this was ventral in 

 position. 



Excisions at levels 7/8, 9/10, 10/11, and 13/14 alone resulted 

 in hypermeric regeneration but the total number of such cases is 

 too small for generalization. Only one extra segment was formed 

 in each case. In these hypermeric worms all the regenerated seg- 

 ments except of course the first were setigerous and clearly out- 

 lined by intersegmental furrows. No half-segments, wedge- 

 shaped or otherwise were found in heads of this type. 



When the anterior end was excised posterior to 17/18 only 

 hypomeric heads of ten to sixteen segments were formed. The 

 number of worms operated on behind this level was too small to 



