132 ELIZABETH HOPKINS DUNN 
of any further duplicating studies. When it was learned later 
that Mrs. M. H.S. Hayes had a considerable series of counts of the 
ventral root nerve fibers of the second cervical nerve for the albino 
rat, postponement of publication was made for more than two 
years, but as her material has not been published it has seemed 
expedient to present my own findings with all due. apologies to 
others. 
With age as the constant factor in each group it seemed wise 
to vary the other factors which might determine the size of the 
largest nerve fibers. The additional factors selected were sex, 
body size as shown by extremes of weight within the limits of 
health, and relationship as shown by conditions within the same 
litter or in different litters. To these ends seven groups of rats 
were selected ranging in age from seven days to two hundred and 
seventy days, or nine months. These two limits were determined 
upon, the first because technical difficulties made accuracy in 
measurement among younger rats rather uncertain and the 
second because it had been found difficult to maintain good health 
and normal weight among laboratory rats after nine months of 
life. Each one of these seven groups was made up of four rats 
of the same age but of as widely varying weights as could be 
secured among healthy animals. The emphasis was laid upon the 
selection of light and heavy females, light and heavy males, thus 
including two females and two males in each group. Further 
information concerning conditions within and without the litter 
was secured by selecting certain groups from one litter and other 
groups from scattered individuals. The four rats of seven days, 
three of fourteen days and the one rat of one hundred and thirty- 
eight days are of one litter. The four rats of the group at thirty- 
six days are of one litter. The three rats of seventy-five days are 
of one litter. The four rats of the group at two hundred and 
seventy days are of one litter. 
To this series of seven groups was added a group of aged rats, 
three males, of widely varying weights but not in good health. 
These rats were about six hundred and forty days of age. 
As previously stated, the medullated nerve fibers of the ventral 
root of the second cervical nerve were selected for study and for 
