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GENETICS: C. C. LITTLE 
THE USE OF VASECTOMIZED MALE MICE AS INDICATORS 
By C. C. Little 
HARVARD MEDICAL SCHOOL 
Communicated by R. Pearl, January 22, 1917 
In the course of certain experiments with mice it became necessary 
to have some way of determining the presence of the ovulation period 
in females which were still virgins. The small size of the external 
genetalia and the minuteness of the changes in them at periods of ovu- 
lation make methods of direct observation difficult if not impossible. 
It was found convenient therefore, to use vasectomized males to 
indicate the physiological condition of the females. For this purpose, 
males sexually mature or nearly so, were etherized and an incision about 6 
mm. long was made, a little to the right of the mid-ventral line in the 
right inguinal region, the hair having been previously clipped in that 
locality. Care was taken to avoid the larger blood vessels in the peri- 
toneum. Through this opening it was possible with a pair of small 
serrated forceps, to lift out first the right and then the left testicle and 
their respective vasa defferentia. A piece of the vasa defferentia, about 
3 mm. in length, was tied off with fine black silk, and the section be- 
tween the ligatures removed. The testicles and vasa defferentia were 
then replaced in the peritoneal cavity and the body wall and the outer 
skin sewed separately. Warm salt solution was used when necessary 
to keep the exposed organs moist. 
In a great majority of cases the operation was entirely successful 
and the mice recovered rapidly. In two weeks these mice were placed 
with females known to have had young between twenty-four and thirty- 
six hours previous. The vasectomized males behaved like normal 
males as regards their sexual instincts, and attempted successfully to 
copulate with the females. In so far as it was possible to observe, 
there was no difference in the process of mating between vasectomized 
and normal males. 
A breeding test on a larger scale was made to determine whether in 
a considerable period of time the vasectomized males would be able to 
fertilize an occasional female. For this purpose virgin females, just 
becoming sexually mature, were placed with vasectomized males and 
were controlled by an equal number of similar females placed with 
normal males. The first twenty males, both vasectomized and normal, 
were allowed to remain with females for only ten days. During this 
time in a total of 106 females, the pens containing vasectomized males 
