904 
Vol XXX, No. 10 
Journal of Agricultural Research 
growth rates of pollen tubes as shown 
by Correns (8), but that a reduction 
should cause the sex ratio to deviate 
much more on the other side of equality 
is hardly explainable upon that basis. 
One other effect which the with¬ 
drawal of the seminal fluid with a cathe¬ 
ter might have had is to carry parts of 
the fluid much farther up into the 
uterus than a natural service would 
have done, but even if this were done to 
a considerable extent there does not 
seem to be any reason to think that 
more of the female-producing than 
male-producing spermatozoa would be 
carried up into the uterus nearer to the 
unfertilized ova. Furthermore, if it 
were only a matter of pushing the sper¬ 
matozoa'far up into the uterus, all the 
litters produced by artificial insemina¬ 
tion should show a very high propor¬ 
tion of females, whereas actually there 
was only a bare majority of females. 
Since there is apparently no physical 
basis for this abnormal sex ratio, judg¬ 
ment must be reserved as to its real 
significance until there are more data 
available. However, it is certain that 
none of the other methods of breeding 
has produced a significant change in the 
sex ratio, for if, instead of equality the 
ratio of 106.9 males be taken as normal, 
the deviations from expectation become 
in general even less than they are when 
based upon equality. 
SEX OF OFFSPRING IN RELATION TO 
LENGTH OF GESTATION PERIOD 
That there is no relation between the 
sex of the offspring and the length of the 
gestation period is shown by Table II. 
Here the nearest whole number of days 
between the hour of breeding and the 
hour of finding the litter is given. 
Since many of the litters may have been 
as much as 12 hours old when they were 
found, it is apparent that while some of 
them may belong in a period one day 
shorter than that in which they are 
placed, none of them belong in a longer 
period. 
That the length of the gestation 
period would vary with the sex ratio 
of the litter is hardly to be expected, 
since nearly every litter contains both 
males and females, and even if the males 
require a longer or a shorter time to 
complete their development still the 
litter must all be born at the same time 
and only the exceptional cases where 
the litter is all of the same sex might 
be expected to show this relation. 
Since the extreme gestation periods as 
shown in Table II do not give extreme 
ratios and since the average gestation 
Table II .—Sex of offspring and length 
of gestation period 
period of the female offspring was less 
than 0.02 days in excess of that of the 
males, it may be concluded that in the 
rabbit the sex of the offspring bears no 
relation to the length of the gestation 
period. There may, however, be some 
such relation in uniparous animals 
where an embryo of one sex during its 
entire course of development is free 
from possible influences from embryos 
of the other sex in the uterus beside it, 
and it is a fairly common belief among 
breeders of horses and cattle that 
there is such a relation in those animals. 
SEX RATIOS OF PROGENIES OF INDI¬ 
VIDUAL MALES AND FEMALES 
That such deviations from a normal 
sex ratio as may exist are not due to 
the progeny of a single male which 
happen to be grouped together is 
shown by Table III, which gives the 
sexes of the progeny of all males which 
have as many as 50 progeny whose sex 
is known. 
Table III .—Sex of progeny of males 
Sire 
274.3. 
301.3. 
316.1 
411.1. 
397.1. 
440.1. 
391.1. 
516.4. 
.534.1. 
20.2. 
20.2 and 21.41 
26.8 and 20.2] 
ILeadI 
experi-j 
l ments i 
Devia¬ 
tion— 
prob¬ 
able 
0.38 
2 .68 
