SEXUAL ACTIVITY OF MALE RABBITS 579 
tremes of the ilium as determined by the calipers. In so far as 
possible an effort was made to have the animals sit with the 
limbs in the natural position while being measured. ‘The hair 
was also clipped from this region of the body in order that it 
might not obscure the point of the bones. 
4. Methods of interpreting weights and measurements 
In order to make the data for different-sized litters more nearly 
comparable, all weight and measurement records are reduced to 
an ‘individual mean’ for each litter for each of the nineteen periods 
of observation. The individual mean for each litter was cal- 
culated by dividing the total weight or total measurements of 
each litter by the number of individuals for each of the nineteen 
periods. From these individual litter means the series of cumu- 
lative growth graphs are constructed. : 
In attempting to compare the growth graphs of rabbits in the 
different service groups, a very perplexing problem arose as to 
how to best compare results in litters that vary so much in num- 
ber of individuals. The number of individuals born in a litter 
is an intensely important factor in influencing the weight of the 
young. Our observations have shown this as did also observa- 
tions of Minot (91, p. 111) on guinea-pigs. His results, based 
upon 351 observations, show that the average birth weight is 
85.5 grams in litters of one, the weight gradually decreased with 
the increase in number of individuals to as low as 52.2 grams 
in litters of eight. 
Another item that makes comparisons of litters in different 
service groups difficult is the fact that litters in the Ist and 5th 
service groups are likely to contain more individuals than those 
from the 15th and 20th services. For this reason the individual 
mean of these advanced service litters is greater and they grew 
faster because of a more generous supply of milk from the mother. 
In this connection we find that King (16, p. 51) discovered that 
in rats ‘“‘body weight at birth indicates the probable capacity of 
the individual for subsequent growth.’ This being the case, 
small litters from the advanced service should grow more rapidly 
than the larger litters from the Ist and 5th services. 
