160 The Journal 
Still a third situation was found to 
give evidence that the smaller yolks 
are male-producing and the larger yolks 
female-producing. It has already been 
mentioned that females which are “ over- 
worked”’ tend, when older, to begin the 
production of females earlier and ear- 
lier in the season. Now a comparison 
of the size of yolks derived from younger 
and older birds has shown that those of 
younger (but mature) birds are smaller 
than those of older birds. This fact has 
been fully demonstrated. 
We have noted three kinds of evi- 
dence that smaller yolks produce males, 
larger yolks females. There is still a 
fourth piece of proof. It is known that 
the very first egg of life, and the first egg 
produced after a long period of rest, 
more frequently produce a female than 
do the first eggs of succeeding pairs or 
clutches. Study of the actual sizes of 
such yolks has shown that there is a 
reversal in size of these first eggs, cor- 
responding to the reversal in sex, so 
that here too the female sex is associated 
with the larger yolk. Yolk size was 
accurately determined in about 10,000 
cases, and the association of smaller 
yolk with male producing germs, and 
larger yolk with female producing germs 
is well established. The conclusion is 
drawn that here two kinds of ova are 
produced, and the kinds may be identi- 
fied by size differences. 
Next, let us recall that in the pigeon 
(probably in all birds) two kinds of eggs 
are produced, but only one kind of 
sperm. There is breeding evidence, in 
addition to the evidence accumulated 
by Dr. Riddle, which would seem to 
establish this fact. The determination 
of sex therefore resides necessarily in 
the egg, and the sperm can be ruled out 
of this discussion. 
THE 
We get back to the heart of the prob- 
lem again. If there are two kinds of 
eggs—one which produces males, and 
the other which produces females—how 
is it that early in the season practically 
only males are produced, and later in 
the season only females? Is there only 
HEART OF THE PROBLEM 
of Heredity 
one kind of egg produced at each period? 
And is the kind reversed in the two 
periods? 
Quite impossible, Dr. Riddle answers. 
‘“We can easily demonstrate, by weigh- 
ing or otherwise measuring them, that 
both kinds of eggs are produced through- 
out. What actually happens is that the 
‘generic cross’ which produces all, or 
nearly all, males in the spring and all, 
or nearly all, females in the autumn, is 
utilizing in the spring a number of 
female-producing ova for the produc- 
tion of males, and in the later season is 
utilizing for the production of females 
ova one-half of which had initial inclina- 
tion for the production of females.” 
That this is true, is shown not only by 
weighing the eggs, but by a number of 
other lines of evidence to be cited later; 
and also “‘by the fact that if the same 
female which threw all males in the 
spring had been mated with another 
bird of her own kind, and made to lay 
eggs at a similar rate, then both males 
and females would certainly have ap- 
peared at all seasons.” It is only 
because we are studying a cross of 
birds, each from a distinct genus, and 
applying the pressure of “overwork,” 
that we are upsetting the normal con- 
ditions enough to see what is going on 
beneath the surface. 
We have now excluded two possible 
explanations of the observed  sex-con- 
trol in pigeons. We have shown that 
it cannot be due to selective fertilization 
by the sperm—for there is only one kind 
of sperm. We have shown that it can 
not be due to selective elimination of 
ova in the ovary—for it is known that 
the two kinds of ova—male-producing 
and female-producing—are being laid 
all the time. 
The next possible objection to the 
supposition that we are dealing with a 
real case of sex-control, would be a 
technical one, alleging that everything 
which happens might be explained as 
differential maturation. To this Dr. 
Riddle answers: 
The maturation would have to be definitely 
differential in (1) the elimination of an X 
chromosome during the spring from one-half 
‘In a paper read before the American Society of Naturalists, Columbus, Ohio, December 
30, 1915. 
