GENETICS: C. C. LITTLE 
187 
had a record of no observed pregnancies, while the controls showed 
seven pregnancies in a total of 98 females. The second series of results 
showed a more definite difference. In eleven pens in which vasecto- 
omized males were allowed to remain for from eighteen to twenty days, 
with a total of 60 females, there were no pregnancies; while control 
males remaining in pens containing 55 females for from eighteen to 
twenty days gave a total of nineteen pregnancies. The chief reason 
for lengthening the time during which the females and males were 
together was the appearance of a communication by Long concerning 
the period of ovulation in mice, which states that the average length 
of time between the first and second ovulations following parturition 
in mice is from seventeen to eighteen days. It became evident there- 
fore, that the first series of experiments in which female mice were 
allowed to remain with the males only ten days, would not be critical, 
in as much as the space between ovulation periods in virgin mice is 
undoubtedly approximately the same as the length of time between the 
first and second ovulations following parturition; and therefore proba- 
bly only about one-half of the females would pass through an ovulation 
period while with the males. The longer period gives a far better test 
as the results show. 
Before the experiments here recorded were made, a number of males 
were operated on and in each case a portion of the vasa defferentia 
3 mm. long was removed. The cut ends, however, were not ligatured. 
Thirty-nine of these males were chloroformed, slightly over two months 
after operation, and were examined to see whether any possible connec- 
tion between the severed vasa defferentia had been reestablished. In 
no case was there any sign of such a connection. In many cases there 
was obvious blockage and distention of the vasa defferentia, apparently 
by retention of substances which would have ordinarily passed through 
them. Certain signs of inflammation and reduction in the size of the 
testicles were also frequently observed. It would be interesting to see 
whether the sexual instincts of vasectomized males remained normal 
after the degenerative changes of the testes and vasa defferentia referred 
to had set in. 
By supplementing observations on the sexual instincts of these males 
with a careful histological examination of the testes it might be possible 
to obtain an additional line of evidence as to whether it was primarily 
the interstitial or the sperm forming cells which influence the secondary 
sexual characters. 
By the use of vasectomized males it will be possible to pick out the 
females which are in a suitable physiological condition for successful 
