180 JOURNAL OF COMPARATIVE NEUROLOGY. 
For clearness, only the totals obtained by adding the 
fibers in the roots as shown in Table III, will be discussed. 
As we see from the above table, the ratios between the 
two roots in 10.3 grams is 1:2.9, while in the mature animal it 
is 1:2.3. This means that the number of the ventral root fibers 
increases more rapidly than that of the dorsal root fibers, con- 
trary to what appears in the frog (BirGE, 1882). The addition 
of the new fibers between 165 and 264 grams is very small, 
and therefore the animal having a body weight of 165 grams 
may be regarded for this purpose as adult. These results 
show that the number of medullated fibers in the ventral roots 
of the spinal nerves of the rat as well asin the dorsal roots 
increase more than 100% with age—the age being indicated 
here by the body-weight. On the other hand, ScHILLeEr (’89), 
who counted the number of the medullated fibers in the oculo- 
motor nerve of the cat at different ages, showed that the nerve 
of the new-born kitten contained 2942 fibers, while that of the 
eighteen months old cat was found to have 3035 fibers. 
Thus, according to him, the number of the fibers added 
between birth and maturity was only 83; or hardly 3% more 
than the number found at birth. If this observation is correct, 
it indicates that the development of these efferent neurones in 
the cat is completed very early. The lack of accordance in the 
observations on the oculomotor nerve of the cat with those on 
the efferent spinal nerves of the rat, demands further study. 
Several years ago, Harpesty (’99), made the important 
observation on the spinal nerves of the frog that the number of 
medullated dorsal root fibers is more numerous near the 
ganglion than at the entrance of the dorsal root into the cord, 
while in the case of the ventral root, the number of medullated 
fibers diminishes from the cord towards the ganglion. This he 
interpreted to mean that the dorsal as well as ventral root fibers 
are growing (medullating) even in an older animal, and there- 
fore as we pass away from the cells of origin, the number of 
mature fibers diminishes. DaLe (’00), who counted the num- 
ber of medullated fibers in the dorsal roots of several spinal 
