BAND: NERVOUS SYSTEM OF LUMBRICIDiG. 121 



All the facts taken together justify the belief, as Hescheler concludes, 

 that the epidermis is an important source of cells that take part in 

 regeneration, and especially of cells that go to the regenerating cord. 



There are other possible sources of the nuclei which resemble those 

 found in the regenerating cord. The alimentary epithelium contains 

 such nuclei, and it is not impossible that it may furnish cells for the 

 regeneration of other organs, but there is no good evidence for believing 

 that it does so to an important extent in the case of the nervous system. 

 There are scattered nuclei among the muscle fibres, and elsewhere, 

 which resemble the epidermal nuclei, and these may also take part in 

 regeneration. 



Three facts justify the conclusion that the cells of the new brain and 

 ganglia are mainly of epidermal origin. (1) In the early stages of re- 

 generation the nerve cord is in more intimate relation with the epider- 

 mis than with any other tissue. (2) The new nervous parts are laid 

 down in a region whose cells (with the exception of the lymph cells and 

 the spindle cells) are doubtless derived mainly from the epidermis. 

 (3) There is an apparent inward shifting of cells from the epidermis to 

 the new cord. 



One possibility remains to be considered. Hescheler finds, in regen- 

 erating worms, abundant mitosis in the ganglia of the old cord back 

 through some fifteen segments, and concludes that the old cord furnishes 

 some material toward I'egeneration. This increase of cells in the old 

 cord has not been commonly found in my preparations. In most of the 

 series that have been carefully studied there was no evidence whatever 

 of mitosis or increase of cells in the old cord. In one or two animals a 

 condition was found similar to that described by Hescheler. In one 

 worm of ten days' regeneration were found several masses of cells pre- 

 senting an appearance unlike anything to be found in a normal gan- 

 glion and containing so many mitotic cells as to indicate a rapid increase. 

 The preparation included nine segments back of the injured segment. 

 The most posterior mass of proliferating cells was in the eighth seg- 

 ment. Several similar masses occurred in ganglia of more anterior 

 segments and occasionally isolated dividing cells were found. 



In the worm from which Figure 14 (Plate 3) is taken, the mass of 

 cells which has already been mentioned (page 116) as lying at the ante- 

 rior end of the old cord is doubtless of the same nature as those found 

 farther back in the other animal. This preparation included only five 

 segments and no similar masses were found in other segments. In 

 appearance the cells of this mass (Plate 4, Figure 18) are precisely like 



