52 Journal of Comparative Neurology and Psychology. 



In the frog at least, that in a Hmited number of nerves an 

 older animal may show fewer cells than one somewhat younger, 

 can be equally well explained as due to the fact that the pro- 

 portional number of neurones apportioned to a given spinal 

 nerve is by no means constant. 



The progress of growth in the frog is evidently different 

 from that in the mammal. The 7 gram frog is no doubt rela- 

 tively more developed than the 10 gram rat and in the rat of 

 10 grams and younger the processes of growth must go on 

 much more rapidly. This I think is indicated in the two tables. 



While the number of ganglion cells may not increase with 

 age in the rat, the evidence though slight is, I think, a little 

 stronger for the frog. It is needless, perhaps, to go into the 

 literature for the support of this view for so far as I am aware, 

 there are no observations which directly maintain it. Buhler 

 ('98) in discussing the cells of the spinal ganglia suggests that 

 the large cells are continually degenerating while the small cells 

 enlarge and replace them. If this were true, the cells would 

 have to multiply or else the number would, on the contrary, 

 •decrease with age. Of the 18 papers that I know of, which, 

 dealing with tissues more or less mature, describe appearances 

 in nerve cells (vertebrate and invertebrate) thought to be con- 

 cerned in the processes of cell division, none of them describe 

 cases of undoubted nerve cells which can be confidently consid- 

 ered the actual process of division. Of the list Lenhossek ('95), 

 Dehler ('95) and Buhler ('98) describe such appearances in 

 the spinal ganglion cells of the frog. 



Sumvia7'y. 



I. Due to variations in the relative number of neurones 

 apportioned to a given spinal nerve, the number in a given nerve 

 may not increase regularly with the increase in body weight but 

 the sum obtained by adding the numbers in a given nerve of 

 the larger specimens, as well as the sum of the several nerves 

 of the larger specimens, is always greater than that of the smaller 

 specimens. 



