128 W. J. MOENKHAUS 
in the vial with a sterile pair and from time to time removing the 
eggs one by one with the point of a needle and placing them on a 
piece of moist filter paper in a separate vial. Usually twenty were 
placed in each vial and some food added for the larvae, should they 
emerge. Inspection of the eggs after twenty-four, forty-eight and 
seventy-two hours would readily reveal the number of eggs that 
had produced larvae. I have laid out thus at a great expense of 
time literally thousands of eggs from many infertile pairs, in 
many cases all the eggs that a given pair produced during the first 
twenty-five days of its life, but I have never seen a single egg that 
had hatched. Eggs of fertile pairs thus laid out will readily hatch 
so that all the larvae will have taken to the food twenty-four hours 
after the eggs are deposited. 
Such infertile pairs copulate frequently and it would seem that 
impregnation should follow. I have never sectioned the eggs 
to see whether spermatozoa enter the eggs or whether they con- 
tain partially developed larvae which fail to hatch. I have, 
however, been able to determine in this strain which of the sexes 
is at fault. This was done in the following manner. After a 
pair by sufficient trial had proven itself infertile, the male was 
mated to a virgin female of a fresh strain that had not been inbred 
and possessed a high degree of fertility, and the female was simi- 
larly mated with a male, usually one whose fertility had been estab- 
lished. Sixty-four such cases were tried and in no case did the 
females fail to produce young and in no case did the males pro- 
duce any although repeated copulations took place. It is evident 
from the foregoing, that, in this strain, the sterility lies exclusively 
in the male and that the female has lost, apparently, nothing in 
fertility. Castle (p. 735) reports, on the other hand, that either 
sex may be sterile. However, Castle took no account of the eggs 
and larvae but merely the production of pupae, so that his steril- 
ity cannot be with certainty compared to mine. It would seem, 
however, that in some strains infertility may be strictly confined 
to the males and in others to both sexes. That sterility is com- 
plete for all males, when it occurs, is shown by both our results. 
b. Degrees of sterility. The foregoing experiments concerned 
themselves with such pairs as were completely sterile. Other pairs 
