REGENERATION 



were united by their anterior ends, only eight segments being 

 removed from each worm, although perfect union took place at 

 first, later one or two new heads generally developed at the region of 

 union (Fig. 52, D). When only one head developed it did not seem 

 to belong to one of the components rather than to the other, and 

 originated in the new tissue that appeared between the two pieces. 

 These experiments, in which the anterior surfaces of two pieces are 

 united, show also that the new head arises between the two pieces 

 most often, if not exclusively, when the union is in the anterior ends 

 of the worms. This corresponds with what is now known in regard 

 to the development of new heads by isolated pieces, since there is less 

 tendency to produce a head the farther posteriorly the cut has been 

 made. At more posterior levels a tail and not a head is often regen- 

 erated, as has been stated, on the anterior cut-surface. This forma- 

 tion of a heteromorphic tail seems to have been suppressed in the 

 pieces united in this region, except in one case, 1 in which it appears, 

 from Joest's account, that a tail probably regenerated, although Joest 

 speaks of it as a head. 



It is more difficult to unite two anterior ends by their posterior 

 cut-surfaces, not because the surfaces refuse to unite, but because the 

 two pieces crawl away from each other and pull apart. In one case, 

 however, union of this sort was brought about. 



In all the combinations that have been so far described, the 

 dorsal and ventral surfaces of both components were kept in the 

 same direction, so that the ventral nerve-cord of one piece came in 

 contact and fused with the nerve-cord in the other piece. Sometimes 

 it may happen that the components are not quite in the same position, 

 and the end of one nerve-cord may fail to abut against the other one. 

 In such cases Joest thinks that regeneration is more apt to take place 

 in the region of union, and he has carried out a series of experiments 

 in which the pieces were intentionally united, so that they are not in 

 corresponding positions. It is found that if one piece is turned so 

 that the nervous system lies 90 degrees, or even 180 degrees (Fig. 52, 

 B), from that of the other piece, the union takes place just as when 

 the pieces have the same orientation, except that the ends of the 

 nerve-cords do not unite. Subsequent regeneration from one or from 

 both components generally takes place in the region of union. 



It is more difficult to unite pieces of different species of worms, 

 yet Joest has succeeded also in making combinations of this sort. 

 One union between the anterior end of Lnmbricns rubellus and the 

 posterior end of AllolobopJiora terrestris was permanent, and the new 

 worm reacted as a single individual, and lived for eight months. 

 Each piece retained its specific characters, and showed no influence 



1 See Joest's Fig. 14. 



