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SEXUAL ACTIVITY OF MALE RABBITS Dio 
males, and by making matings at such a time as to secure all 
types of litters at the samme season of the year, as far as pos- 
sible, we hoped to overcome many possible sources of error. 
However, as the experiment proceeded it was found impossible 
to apply these corrections absolutely and we are thus not fully 
justified in comparing litters in growth in body weight and in 
mean dimension and assuming that any consistent differences 
are due to the number of services performed by the males. We 
should not overlook the slightly better opportunities offered the 
15th- and 20th-service litters, a considerable proportion of 
which were born during the latter part of the experiment and 
were produced when the females were all mature. 
2. Records kept 
The following records were kept: Date of breeding, pedigree, 
date of the next probable heat period—fifteen days after breed- 
ing; actual date of parturition; number in litter; number born 
dead; sex of offspring; individual weight of offgpring on day of 
birth and for each five days thereafter up to ninety days; head 
length and breadth through extremes of ilium, taken at the 
same time as the weights; date of weaning; color, and mortality 
record. 
3. Weighing and measuring 
Breeding records were kept for each female so that it was pos- 
sible to weigh each litter on the actual day of birth. At this 
time each litter was given a number which was the same as the 
number of the matings, and each individual was given an indi- 
vidual number and marked in such a way as to be easily dis- 
tinguished from litter mates by color description or by clipping 
ears and tail. The individual weight records were kept each 
five days until the litter reached the age of ninety days. 
The desirability of continuing all records to the full maturity 
of the progeny is very apparent. As the work was handled, forty 
or fifty animals were often weighed and measured on a single day 
and, with the other routine work of the experiment, entailed a 
