io8 Journal of Comparative Neurology. 



(2). In a second paper (1901, II), the writer made the 

 following suggestion concerning the possibility of the division 

 of the nerve cells in an adult animal, "We can only say, at 

 present, concerning the division problem that the nerve cells in 

 vertebrates, as well as invertebrates, have the centrosome and 

 the sphere, which are regarded as the dynamic centers of the 

 mitotic divisions, and, further, that this centrosome is able to 

 take the first steps of division under certain forms of stimula- 

 tion, as has been observed by some investigators ; but in the 

 normal state the centrosome in an adult cell presents slight mor- 

 phological differences from that of the embryonic cell, which 

 we interpret as the beginning of degeneration." The writer 

 said further, "In order to give a positive answer to the question 

 (division-problem) above mentioned, it seems to me that the 

 only safe and reliable method consists in counting the nerve 

 cells in the spinal ganglia of a given species of animal at differ- 

 ent ages, and thus determining whether there is any increase in 

 their number." 



The present investigation consists of a series of enumera- 

 tions of the spinal ganglion cells and dorsal root fibers, under- 

 taken with a view to settling these questions. 



II. Material and Technique Employed for the Present 



Investigation. 



Male rats having a body weight of 10.3, 24.5, 68.5 and 

 167 grams respectively, were employed. The exact ages of 

 these rats are not known but approximately their weights in 

 grams represents their ages, in days. The ganglia examined 

 were the VI cervical, IV thoracic, and II lumbar. As soon as 

 the dorsal roots with the corresponding ganglia were removed 

 from the cord they were placed on card-board without disturb- 

 ing the natural length of the nerve roots and preserved in i ^ 

 osmic acid solution for 24 hours. After this the specimens 

 were detached from the card-board and washed in running water 

 for 6 hours. Afterward, the specimens were carried through 

 the graded alcohoLs and embedded in paraffin according to the 

 usual procedure. 



