122 bulletin: museum of compakative zoology. 



those of the more posterior groups of the other worm, and uulike any 

 found in the near-by cicatrix. These facts, and the position of the mass 

 well within the limit of the old cord, make it scarcely to be doubted 

 that the mass owes its origin to the increase of cells of the old cord, 

 rather than to any cells which may have wandered in from the cicatrix. 



It is the occurrence of masses of actively dividing cells, rather than 

 the character of the individual cells or nuclei, that distinguishes this from 

 the condition found in the cord of a normal worm. The nuclei of these 

 cells differ in no marked way from nuclei which may be found in the gan- 

 glia of uuinjured worms. They are smaller than the nuclei of the large 

 nerve cells, but they show the same structure as to chromatic material 

 and the presence of a single large nucleolus. Figure 57 (Plate 8) shows 

 some cicatricial cells of the epidermal type, but the same drawing would 

 represent equally faithfully a group of cells from one of these masses in 

 the old cord. The nuclei differ from any found in the normal cord 

 mainly in beiug more or less elongated. In Figure 18 it can be seen 

 that the nuclei iu the thin posterior end of the mass are more elongated 

 than those in the thicker anterior part of it. This condition suggests a 

 mechanical deformation of the nuclei as a result of the rapidly growing 

 mass pushing its way back into the tissue of the cord. There was scant 

 cytoplasm about these nuclei and no definite cell outlines, as is also the 

 case in the nuclei of Figure 57. 



The absence of this cell increase in the old cord of many animals 

 shows that it is not a necessary feature of the regenerative process. 

 The presence of one of these masses of cells exactly at the region of 

 injury, as in Figure 18, makes it probable that it provides material for 

 regeneration. It can hardly be supposed that an increase of cells in 

 ganglia situated five or ten segments back of the region of injury has 

 anything to do with the actual regeneration of new ganglia anterior to 

 the old cord. 



5. Histology of the Nervous Fundaments. 



Having considered the origin of the cells which constitute the funda- 

 ments of the brain and new ganglia, I now propose to consider certain 

 questions pertaining to the development of the nerve cells from the cells 

 of these fundaments. The main purpose at this point is to determine if 

 any polar structures of a mitotic cell pass into the resting cell, persisting 

 through all the stages of growth and differentiation to become, or give 

 rise to, the centred system of the mature nerve cell. It is therefore 

 necessary to examine the mitosis, and especially the later stages of it, 



