333 Journal op Comparative Neurology. 



cord thus moulded, is given for the frogs of several sizes, and 

 assuming the specific gravity of the nerve substance to be unity 

 (it is probably about 1.03), the square root gives the length in 

 millimeters of each side of the mass. For under the conditions 

 stated I cubic millimeter weighs i milligram. 



Since the weight of the average millimeter increases with 

 the increase in the size of the frog, it is plain that enlargement 

 both in the brain and the spinal cord is taking place in planes at 

 right angles to the long axis, as shown in columns 3 and 5, 

 Table 8. 



If the enlargement had been proportional along all the 

 axes, then (the natural geometric form of the brain and cord re- 

 maining the same, as is practically does), the weight relations 

 in any two frogs would be proportional to the cubes of the re- 

 spective long axes of their central nervous system. 



Applying this test to the limiting cases in Table 8, we ob- 

 tain the following : 



Body weight. Length of brain. Length of spinal 



mm. cord. mm. 



1.32 8.4 8.0 

 3»35o ^2f. ^Ll 



On comparing the cubes of these respective lengths, it ap- 

 pears that the increase should be 1 5 fold for the brain and 40 

 fold, for the spinal cord if the enlargement along all the axes 

 had been proportional. The actual weight relations as shown 

 in Table 2, indicate, however, an increase of only 8 fold for the 

 brain and 1 2 fold for the spinal cord. From these data several 

 conclusions can be drawn. First that the increase in the long 

 axis of the brain and spinal cord is more rapid than in the other 

 axes, and second, that enlargement in planes at right angles to 

 the long axis is more slowly accomplished in the brain than in 

 the spinal cord. Moreover, in both localities, the latter change 

 is more marked during the early stages of growth. The accom- 

 panying Chart 2, based on the length of the cord and length of 

 one side of the corresponding millimeter segment, shows these 

 relations better than the table. 



