898 
Journal of Agricultural Research 
Vol. XXX, No. 10 
cent of that of the female-producing 
spermatozoon. His measurements of 
head lengths of mature spermatozoa 
also show a very distinct dimorphism, 
the smaller mode being 82.5 per cent 
of that of the larger one, which is a 
very close check with the calculated 
difference when it is considered that he 
was measuring only one dimension, 
whereas volume would depend upon 
three dimensions. The main facts as 
to the suitability of the animals used 
are contained in the statement that 
rabbits are excellent material in every 
respect but one—their chromosome 
composition—and that swine are ex¬ 
cellent material chiefly in that one 
respect alone. 
PLAN OF EXPERIMENT 
The plan of the breeding experiment 
was to secure normal rabbit sperma- 
toza immersed in the liquid which 
naturally surrounds them in a normal 
copulation, then to subject them to 
centrifugal treatment, then to inject 
various portions of the centrifuged 
liquid into the vagina or uterus of the 
female rabbits which were in heat, and 
finally to observe the sexes of the off¬ 
spring resulting from these different 
kinds of artificial insemination. 
FEMALE BEHAVIOR 
It was necessary to know whether 
a female was in heat, and since there 
is no more certain sign of oestrus in the 
doe rabbit than her willingness to 
copulate with the buck, a number of 
sterile males were provided to act as 
teasers or test males to determine 
whether a female was in heat and yet 
to avoid having her become pregnant 
to a natural service. There are other 
indications of oestrus, more or less 
reliable, such as the degree of conges¬ 
tion or swelling of the vulva, mounting 
the other females, etc., but none of 
these indications was completely re¬ 
liable for all the female rabbits. Each 
female had a type of behavior, when 
in heat, to which she adhered rather 
consistently, and it was possible after 
becoming acquainted with her indi¬ 
vidual record to predict rather accu¬ 
rately whether she was in heat without 
actually placing her with the male. 
However, the method of placing the 
females with the test males was 
always relied upon as being more 
accurate and almost as convenient 
as any other. If the doe was in heat, 
she usually accepted the buck almost 
immediately when placed with him, 
and if she did not accept him in the 
first minute or two there was very 
little probability of her accepting him 
at all, no matter how long he was 
allowed to tease her; but there were 
exceptions to this, and very rarely a 
doe that had refused one buck for as 
much as 10 or 15 minutes would accept 
another almost immediately when 
placed with him. 
TEST MALES 
The test males were made sterile by 
the operation of vasectomy, which is 
very easily performed on the rabbit. 
The test males varied in their behavior 
and in the ardency of their sexual 
desire, but so did the normal males, 
and although no method of making a 
measurement of this feature was 
devised, close observation leads to the 
conviction that the males which were 
not vasectomized until they were 
sexually mature were no less ardent 
and no less active sexually than the 
perfectly normal males. Whether the 
males vasectomized before they were 
sexually mature were affected by the 
operation at all remains somewhat 
uncertain. Some of them did develop 
normally and became quite ardent, 
but there were others which always 
remained rather weakly and never 
became ardent enough to be satisfac¬ 
tory test males. However, this was 
also true of some of the untreated 
males. There was nothing in this 
work to prove or even to indicate very 
strongly that the operation of vasectomy 
produces any psychic changes (40). 
RECOVERY OF SEMEN 
Ejaculation can not be induced in 
the rabbit by artificial stimulation and 
the use of a vaginal sponge or a condom 
is not practical (£7), so the seminal 
fluid was recovered by allowing a buck 
to serve a female normally and then 
recovering as much fluid as could be 
obtained from vagina and uterus by 
means of a catheter and rubber bulb. 
This method seems to be the only prac¬ 
tical one for an experiment of this 
kind on rabbits, and moreover it is 
simple and easily operated and the 
amount of fluid which can be recovered 
is large; but it has the disadvantage 
that any secretions or cellular residues 
or bacteria within the vagina are also 
drawn into the catheter and any effect 
which they may produce upon the 
spermatozoa can not be separated 
out. Moreover, an infection is readily 
spread from female to female by this 
method, since the fluid can not be 
sterilized without killing the sperma- 
