50 Journal of Comparative Neurology and Psychology. 



in the frog (col. G, Table IV) do not vary as much as they do 

 in the rat. In fact, the extremes, the specimen of 7 grams and 

 that of 63 grams, show similar ratios between their ventral and 

 dorsal root fibers. The group or average ratios, however, indi- 

 cate the more rapid increase of ventral root fibers mentioned 

 above. 



Table IV further corroborates a relation indicated in the 

 foregoing tables. Column D indicates that in the sums of the 

 trunks and dorsal branches combined there is a gain of 30.7 

 fibers for each gram gained in weight. This is a greater gain 

 than in either root or in both roots combined. In proportion 

 to the number of fibers contained in them, the gain in the 

 trunks and dorsal branches is only 37.7%, or less than the per- 

 centage gain in the ventral roots. But the percentage rate is 

 made up in the dorsal branches considered separately (col. E). 

 Here the gain is about 59%. This again suggests that some 

 correlation exists between the dorsal branches and the distal ex- 

 cess, for as shown, the distal excess increases at a more rapid 

 rate than the fibers in either the dorsal root, the ventral, or in 

 the trunk and dorsal branches. 



The question of the ganglion cells is rather puzzling. The 

 tenet more usually held is that in the vertebrates the number of 

 nerve cells is fixed at quite an early stage in the development. 

 This belief applied to the frog precludes an increase in the num- 

 ber of spinal ganglion cells even at the youngest stages employed 

 here. Yet, when the numbers of ganglion cells found in the 

 different specimens are considered in the same way as the fibers 

 in the nerves which admittedly increase with growth, the cells 

 appear to undergo, in their sum relations at least, variations 

 somewhat similar to those of the fibers. As arranged in Table 

 IV (col. A) it is indicated that for each gram the frog increases 

 in weight, there is a gain of 27.8 cells in the spinal ganglia, or, 

 as the average specimen increases from 142 grams to 51.6 

 grams, the cells undergo an increase of 20.9%. This percent- 

 age increase is less than that of the fibers as must be the case, 

 but it is surprising that it appears so nearly equal to it. 



It is seen (col. F) that in the younger specimens the num- 



