550 HENRY H. DONALDSON AND G. NAGASAKA 
of its axon process, the process which runs in the dorsal spinal 
root. The change must therefore be associated with the distal 
process of the ganglion cell, which is to be considered physio- 
logically as a dendrite, bringing impulses into the cell body and 
in some way stimulating not only its growth as a whole, but the 
progressive change in the nucleus-plasma relation, which appears 
to be an adaptation to maintain the discriminative sensibility 
despite the increase in the skin area over which a given fiber is 
necessarily distributed. 
CONCLUSIONS 
This study was made to determine whether in the albino rat 
the increase in the diameter of the cell body of a neuron and in the 
diameter of the fiber, or fibers, coming from it ceased at the 
same time. It was therefore a study of the later phases of the 
growth of the neuron. 
The material was taken from the seventh cervical segment of 
the spinal cord with the accompanying nerves, and measurements 
were made on the largest fibers in the dorsal and ventral roots and 
in the nerve just distal to the ganglion cells (all fixed in osmic 
acid) and also on the spinal ganglion cells and on the ventro- 
lateral group of large efferent cells in the ventral horn of the 
spinal cord (both fixed in Bouin’s fluid). 
Under these conditions of preparation, and as shown in tables 
7 to 18— 
1. All of the nerve fibers enlarge to the same extent, showing 
(tables 7 and 8) in the heaviest group, diameters 1.7 greater 
than in the lightest group of rats which weighed 29 grams and 
were twenty-four days old. 
2. The axis-sheath relation in these fibers was such that the 
area of the axis in the youngest groups was about 40 per cent of the 
area of the entire fiber, but this area increases with the increasing 
age (and size) of the rat until it becomes about 50 per cent in the 
largest animals (Table 10). This is in agreement with the results 
of previous studies in which the same methods of measurement 
were used, and shows that the axis has grown in diameter a 
trifle more rapidly than the entire fiber. 
