154 ELIZABETH HOPKINS DUNN 
In the interpretation of all investigations on the albino rat 
much gratitude is due to Professor H. H. Donaldson now of The 
Wistar Institute of Anatomy and Biology at Philadelphia, for 
his preliminary studies upon the rat. Growth questions in gen- 
eral are of especial value to us with reference to human young. 
Professor Donaldson over a long period of years, more than ten 
to my knowledge, has been inspiring a series of studies of a more 
and more critical nature upon the albino rat as preliminary and 
comparative studies to those on the human body. An outline 
of some of his plans accomplished and projected is to be found in 
the President’s address before the Philadelphia Neurological 
Society (Donaldson, 711). 
While the present investigation was undertaken independently, 
it owes much for its interpretation to the solid and illuminating 
researches just mentioned. It is interesting to find a laboratory 
animal which in so short a lifetime as three years so conveniently 
reproduces many of the conditions found in the human being. 
because of this recapitulation, growth conditions in the peripheral 
nervous system of the albino rat have the added value of aiding 
in the interpretation of anatomical and functional conditions in 
the human body. 
CONCLUSIONS : 
During the period of rapid growth, shown in the albino rat in 
the accompanying tables from the seventh to the thirty-sixth 
day, there is a parallel increase of the number of medullated nerve 
fibers in the ventral root of the second cervical nerve. When the 
sexes are considered separately, the heavier individual at each 
age has the greater number of medullated nerve fibers. Neither 
of these relations is so definitely marked among the adult rats. 
In this series of observations there is found a continuous in- 
crease in the size of both the medullated nerve fibers and their 
axis cylinders to nine months of age. Among the old rats about 
six hundred and forty days of age, there is a noticeable decrease 
in size from that of the nine month rat both in the nerve fiber 
and its axis cylinder. 
