Success in Controlling Sex 
become more and more exhausted, the 
proportion of females produced from 
their eggs becomes higher. Younger 
birds not previously “‘overworked”’ are 
not easily exhausted in this manner; but 
previously “overworked” old birds, 
under continued ‘‘overwork,’’ will cease 
the production of male offspring ear- 
lier in the spring or summer than did 
these same birds during previous years; 
and from then on to the end of the egg- 
laying period, their eggs will hatch out 
all or nearly all females. 
Now everyone knows that this is not 
what usually happens among. birds. 
There are two unusual situations or 
elements involved in obtaining these 
results: first, a wide cross; and second, 
an enforced increase of ‘reproductive 
overwork.” In ordinary matings of 
pigeons and of other fowls, the two sexes 
will hatch out in about equal numbers 
at any time of year. What is the mean- 
ing of this production, first of all males, 
later of all females, with the “over- 
worked” pigeon in crosses of two dif- 
ferent genera? 
There are several conceivable ex- 
planations. First, it may be true sex- 
reversal—eggs which were destined to 
produce one sex may have been forced 
to produce the opposite sex. If that 
be the real explanation, we are going to 
get an insight into the nature of sex and 
the methods of controlling it. But it is 
also conceivable that nothing so sensa- 
tional has happened. Perhaps male- 
producing and female-producing germs 
were formed in the usual manner, but 
for some reason only one kind was fer- 
tilized. Or perhaps one kind of germ 
died in the ovary, so that all the off- 
spring had to be of the sex represented 
by the other kind of germ, which sur- 
vived. Or perhaps there is a mechan- 
ical internal change in the female-pro- 
ducing germs which turned them into 
male-producing germs. 
In this last paragraph is summed up 
the whole of the problem which faced 
Dr. Whitman and Dr. Riddle. To the 
biologists, it is a much bigger problem 
than it may appear to be on its face, for 
they know that ordinarily two kinds of 
germs are produced, one of which will 
give rise to males and the other to 
159 
females, and it puts a very heavy strain 
on many accepted theories, to believe 
that one of these kinds could be made, 
by pressure upon the germ during its 
growth period, to produce contrary 
results to what the normal mechanism 
does. A change in the internal struc- 
ture of the cell involving chromosomal 
change, or elimination, could perhaps, 
on current views, be accepted without 
great difficulty; but if Dr. Riddle 
demonstrates that he has changed the 
sex-value of the cell without a corre- 
sponding change or elimination of the 
chromosome numbers, he will find, as 
he is certainly well aware, that biolo- 
gists here require very rigid proofs. 
TEN LINES OF EVIDENCE 
Ten different lines of evidence, or 
correlations with the breeding results, 
have been developed from the work of 
Dr. Whitman and Dr. Riddle, and they 
all seem to point the same way. To 
Dr. Riddle, only one conclusion is 
possible from them—namniely, that sex 
has actually been reversed, that male 
offspring have been hatched from fe- 
male-producing germs, and vice versa. 
The first correlation established re- 
sults from a study of the size of the ova, 
that is, the yolks freed from shell and 
albumen or “white.” The yolks of 
late summer and autumn, those which 
produce all or mostly females, are dis- 
tinctly larger than those of the early 
season, which produce males. The 
change in size is gradual, and consider- 
able. 
The pigeon regularly lays a clutch, 
two eggs, at an interval of a day or two 
apart. It was learned that the first 
egg of the clutch, in this experimental 
breeding of pure, non-hybrid females 
was rather regularly—there are excep- 
tions—smaller than the second. Whit- 
man had already shown that in the wild 
species with which he worked males 
predominate in hatches from the first 
eggs of clutches, and females from the 
second. So the conclusion was possi- 
ble that males usually come from 
smaller eggs, both for season, and for 
egg of clutch; females from larger eggs, 
the larger of the season and the larger of 
the clutch. 
