116 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



epidermis, between these points, is considerably thinner than the old. 

 Underlying the new epidermis is a solid cicatricial mass of considerable 

 thickness. The circular and longitudinal muscle layers are seen to end 

 abruptly against the inner surface of the cicatricial mass. The anterior 

 end of the alimentary canal is closed by the cicatrix and may remain 

 closed for several weeks. The nerve cord also ends abruptly against the 

 cicatricial mass. Even after the regenerating part of the cord is well 

 established, the limit of the old cord is sharply indicated by the end of 

 the old sheath, the inner homogeneous layer of which generally curls 

 outward and backward a little at its cut end. In the preparation from 

 which Figure 14 is taken, a more or less lens-shaped mass of newly 

 formed cells lay near the end of the old cord. This is better seen in 

 Figure 18 (Plate 4), which shows the end of the cord drawn to a larger 

 scale, — IGO diameters. Two mitotic cells (Figure 18, cl. mit.) are seen in 

 the lens-shaped mass. The origin and nature of them will be discussed 

 later (page 121). 



Figure 15 represents a parasagittal section through the anterior end 

 of a worm after sixteen days' regeneration. The regenerated parts are, 

 as in the younger stage, included between two asterisks. The regen- 

 erated epidermis remains thinner than the old epidermis. A thin layer 

 of circular (mu. crcJ) and of longitudinal muscle fibres {mu. IgJ) has 

 been laid down in the conical regenerated end. The end of the old 

 nerve cord is sharply marked by the extent of the sheath (tuj) and the 

 abrupt termination of the mass of ganglion cells. Extending forward 

 from the end of the old cord is a fibre tract, which runs out to the apex 

 of the cone, lying ventral to the alimentary canal. The fibres become 

 less distinct anteriorly, and are lost in a mass of cells underlying the 

 epidermis. Along the ventral border of this fibre bundle is a thin layer 

 of cells {gn. nov.) whence are to be derived the cells of the regenerated 

 ganglia. 



Figure 16 is from a parasagittal section of a worm after twenty-four 

 days' regeneration. The regenerated pai'ts are included between two 

 asterisks. There is much variation as to the relative times of appear- 

 ance of the regenerated organs. In Figure 15 (16 days) the regenerated 

 muscle layers are well established. In Figure 16 (24 days) only a few 

 longitudinal muscle fibres in the ventral part of the regenerated cone 

 are to be seen. 



In the sixteen-day preparation (Figure 15) the alimentary canal does 

 not open to the exterior. In Figure 16 (24 days) the breaking through 

 of the alimentary canal to the exterior is nearly, if not quite, accom- 



