152 bulletin: museum of comparative zoology. 



2. The epidermis is the chief source of cells which give rise to the 

 new nervous parts. 



3. In the old cord no evidence of mitosis is found among the large 

 pear-shaped cells with processes. 



4. In some cases, masses of actively dividing cells are found in 

 ganglia many segments back of the region of injury. These masses 

 probably owe their origin to certain small indifferent cells which 

 have retained their embryonic character since the development of 

 the cord. 



These cell masses can take no part in the regeneration of ganglia 

 anterior to the region of injury, except when such a mass arises at the 

 place of injury. 



The stimulus due to the injury is not restricted to the segment 

 injured, but may cause mitosis among cells of embryonic character 

 in ganglia remote from the region of injury. 



5. The regeneration of ganglia is preceded by a forward growth of 

 fibres from the cut end of the old cord. Cells of epidermal origin 

 accumulate ventrally and laterally about this new fibre tract to form 

 the fundament of the new ganglia. 



The fibre bundle divides to encircle the alimentary canal. At a 

 region dorsal to the canal, it becomes associated with a mass of cells 

 lying dorsally and posteriorly to the fibre mass, and, together with 

 these, constitutes the brain fundament. 



6. The deeper cells of the nervous fundaments are the first to become 

 differentiated into nerve cells. The more superficial cells long retain 

 their embryonic character, continuing to divide actively after the deeper 

 cells have become typical pear-shaped ganglion cells. 



7. About the posterior dorsal surface of the normal brain there are 

 some cells with scant cytoplasm, and lacking nerve processes. These 

 are doubtless cells which, in the development of the brain, stopped 

 dividing without becoming differentiated into nerve cells, retaining their 

 embryonic character, perhaps for purposes of regeneration in case of 

 injury. 



8. In mitosis among cells of the nervous fundaments of about five 

 weeks' regeneratio.n, the following peculiarities are to be observed : — 



a. Throughout the process of mitosis the body of the cell outside 

 the limits of the spindle figure is practically homogeneous and clear. 



b. A well defined cell membrane, formed during the prophase, be- 

 comes constricted in the telophase, and an equatorial membrane is 

 formed between the daughter cells. During the later plmses the old 



