38 JouRNAL oF ComPARATIVE NEUROLOGY. 
apparently dividing are remnants of imperfect division of em- 
bryonal germ cells which have become functional in such a con- 
dition without any further morphological changes, or whether, 
on the contrary, they are formed by the division, either mitotic 
or amitotic, of functional adult cells under special conditions. 
If the foregoing supposition is true, then the phenomenon of 
the continuity of the nerve cell bodies does not prove the divis- 
ion of the nerve cells in a matured condition. Since at the 
present moment there is not the slightest direct evidence for 
the amitotic division of nerve cells, we are justified in de- 
manding that good evidence of mitosis as shown by the condi- 
tion of the nuclear substance, be brought forward—if these 
cells are to be considered as actually dividing. This has not 
been done. 
In order to give a positive answer to the questions above 
mentioned, it seems to me that the only safe and reliable meth- 
od consists in counting the nerve cells in the spinal ganglia 
of a given species of animal at different ages and thus deter- 
mining whether there is any increase in their number. The 
present writer is trying this method under the direction of PRor. 
Dona.pson, and hopes to report in the near future. 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 division and further that this centrosome is able to 
take the first steps of division under a certain forms of stimula- 
tion, as has been observed by some investigators; but in the 
normal state the centrosome in an adult nerve cell presents 
slight morphological differences from that of the embryonic 
cell, which we interpret as the beginning of degeneration. 
V. SUMMARY. 
(1). Several classes of nerve cells in the young rat, as 
well as adult rat, have a centrosome and attraction sphere. 
(2). The centrosome in the young rat is more easily dis- 
tinguished from the rest of the structures than that in the adult. 
SS Se ee 
