May 15,1925 Possibility of Sex Control by Artificial Insemination 907 
natural service with semen removed or 
to the total centrifuged—that is, the 
apparent conclusion that centrifuging 
increases the chances of pregnancy over 
insemination without centrifuging is 
probably due only to the harmful treat¬ 
ment given the uncentrifuged fluid. 
Just why dilution of the liquid in¬ 
creased the chances of pregnancy is 
not known; the Ringer’s solution may 
have stimulated the spermatozoa to 
greater activity or in some other way 
may have made fertilization more easy. 
Probably the absolute percentage of 
success should be higher in nearly every 
case, because some barren females 
were used which were not known to be 
barren and some females died or were 
sold while still pregnant. 
The general conclusion can be drawn 
that spermatozoa retain their activity 
and fertilizing ability under more ad¬ 
verse conditions than is popularly sup¬ 
posed. Being kept in an ice box for 
eight days did not completely destroy 
their motility. Neither did the un¬ 
usual handling involved in centrifuging 
and artificial insemination destroy their 
motility. Whether motility and fer¬ 
tilizing ability are the same thing is 
another question; but even though 
they are not, many young are produced 
from centrifuged spermatozoa and 
many were produced from spermatozoa 
exposed to low temperature for periods 
up to 24 hours. The only factor found 
to injure seriously the activity of the 
spermatozoa was the presence of urine 
in the liquid, and no experiments were 
made to determine whether this tox¬ 
icity was due to acidity or to a toxic 
action of some other substance in the 
urine. 
MONTHLY DISTRIBUTION OF LITTERS 
BORN IN LAST 10 YEARS 
The breeding season of the rabbit 
under natural conditions is usually 
given as in the late winter or early 
spring. Table IX gives the number of 
litters by months which were born in 
the last 10 years in this flock. 
These figures can be regarded only 
as giving an approximation to the true 
breeding season because so many 
other factors varied. Thus the breed¬ 
ing flock was much larger at some times 
than at others. Also the breeding 
work was pushed more vigorously 
some months than others, and during 
at least 3 years no breeding was done 
during July and August. The figures 
are presented as tending to support 
the idea that the breeding season is 
in the spring and late winter but deny¬ 
ing that breeding is confined to that 
season. 
Table IX .—Monthly distribution of lit¬ 
ters born in last 10 years 
Num¬ 
Num¬ 
Month 
ber of 
Month 
ber of 
litters 
litters 
January _ 
28 
July_ 
29 
February_ 
44 
August_ . 
21 
March_ 
— 75 
September.. 
14 
April_ 
66 
October_ 
_ 15 
May- - 
89 
November.. 
21 
June_ 
49 
; December... 
40 
OVULATION 
As quoted by Marshall (29 ), Heape 
(17) maintains that ovulation in the 
rabbit is dependent upon the stimulus 
of coition and can not be induced by 
artificial stimulation, but Weil (41) 
maintains that ovulation may take 
place without coition just after the 
delivery of a litter, and Iwanoff 
(19, 20) induced pregnancy by artificial 
insemination of rabbits just after 
parturition. That neither of those 
views goes far enough is shown by the 
four litters produced in these experi¬ 
ments by artificial insemination of 
females which had neither delivered a 
litter nor copulated within less than 
13 days previous and were not even 
placed with a male for 10 days follow¬ 
ing. A fifth litter was produced by a 
female which had been inseminated 
the day after she had produced a litter. 
She was evidently in heat but was not 
allowed to copulate. This is a parallel 
case to those of Iwanoff. It is certain 
that ovulation can take place without 
the stimulus of coition, but it is not 
yet certain that ovulation proceeds 
without any psychic stimulus from 
the courtship of the male as it does in 
the case of the cow. It may be that 
the stimulus of courtship is necessary, 
as it is in pigeons, even though it may 
be two females which court each other. 
In the case of these four litters the 
females were placed with the test 
males and showed signs of heat, but 
were removed before they could copu¬ 
late; hence the physic stimulus of 
courtship was present; but that it was 
necessary seems improbable, since the 
females often show signs of heat 
immediately upon being placed with 
other females, mounting them and 
sometimes even having an orgasm 
when mounted, although no male is 
near and the female in heat has been 
alone in her hutch for 24 hours. 
