SECOND CERVICAL’ NERVE OF THE RAT Lae 
A comparison of the records for the approximate ages shows 
that, while the individuals of the present inquiry, are not the 
lightest or the heaviest which might be obtained, they vary in a 
satisfactory degree from the averages given for males and females 
of comparable ages. I am not able to state the relationships of 
the Watson groups of rats but in my own groups of the younger 
rats a number of litters was included for each age. 
The young of any given litter tend at birth to be of like weights 
and, so far as the argument can be drawn from records of the 
members of one litter, such related rats if differing initially in 
weight tend to approach each other in weight as growth goes on, 
if the conditions of growth are favorable (Dunn, ’08). 
Donaldson (08, p. 353) says, ‘“‘It is a familiar fact that rats 
even of the same litter and reared together grow very differently 
and therefore at the same age may have widely different body 
weights.’’ Unfortunately we have no published data to show the 
initial body weight for related rats which differ so greatly at 
maturity. It appears, however, that unrelated rats and those 
reared under different conditions exhibit less uniformity of growth. 
The present findings suggest that these variations from the aver- 
age may make themselves apparent in the nervous system as 
well as in the body weight. ‘This is shown in the group at one 
hundred and eighty days of unrelated rats which, while fitting 
into the scheme for body weight, give numbers of fibers which are 
less than might be expected. 
In all the findings which refer to weight the females must be 
considered separately from the males, at least after sexual matur- 
ity, since the growth curves for body weight differ greatly. For 
the curves showing this, reference may be made to Donaldson’s 
papers, chiefly the collaboration with Watson and Dunn (’06). 
The body weight for the male rat increases for a longer period than 
that for the female and the average body weight for the adult 
male is considerably greater than that for the adult female of the 
same age. 
The findings for this series of albino rats group themselves under 
two heads, first, the factors influencing the number of medullated 
nerve fibers, and second, those influencing the size of the nerve 
