134 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



8. Conclusions. 



1. Two normal regenerative pi^ocesses are to be distinguished in Dero 

 (1) that by which new segments are formed, limited to the region imme- 

 diately in front of the anal segment, and (2) the formation of the bud- 

 ding zone in segments which have been derived from (1). The first 

 process gives rise to undifferentiated materials from which the bud-zone 

 is produced ; the bud-zone, in turn, produces a segment-forming zone. 

 These two processes are so related in this species that budding may be 

 looked upon as a specialized method of segment formation. 



2. The sexual gonads in Dero vaga arise upon the posterior walls of dis- 

 sepiments iii/iv and iv/v, in individuals known to be produced asexually. 

 The dissepiment iii/iv, on which the testes occur, is produced de novo 

 in the budding process, and dissepiment iv/v is the bounding dis- 

 sepiment of the budding segment. These facts show a close relation 

 betweeu the sexual elements and the new structures formed from the 

 indifferent cell masses during the budding process, and suggest that 

 both are referable to the preanal segment-forming region as their 

 source. 



3. The budding zone in Dero is formed, and division takes place mid- 

 way between dissepiments, i. e. in the middle of the segment, if the dis- 

 sepiments mark the boundaries of segments. Semper ('76) states that 

 in Nais barbata and Xais proboscidea budding occurs between two old 

 segments. Von Kennel ('82) says that the bud is formed immediately 

 behind a dissepiment in Ctenodrilus pardalis. Von Bock ('97) describes 

 the bud as forming between segments in Chsetogaster. His Fig. 14, Taf. 

 VI. illustrates this and shows the dissepiment splitting. It appears to 

 me, however, desirable that the method of the formation of new segments 

 in Chpetogaster be more fully made out. It seems to me that the split- 

 ting of the dissepiment may possibly be a part of the process of the 

 formation of new segments rather than of the immediate production of 

 the more specialized bud-zone, and that the separation occurs, after all, 

 in the middle of this newly formed segment. The appearance of the gut 

 in Figure 14 would suggest this interpretation. We know that new 

 segments are formed in these regions in Chaetogaster, and it is not 

 clear how much of this precedes and how much follows the budding 

 process. 





