Riddle's work on pigeons [5,6] brings us much nearer to man, and
suggests the results noted by both Goldschmidt and Lillie. As in the
Free-Martin cattle, there is an apparent reversal of the sex
predisposition of the fertilized egg. As in the gypsy moths, different
grades of intersexes were observed. In the pigeons, it was found that
more yolk material tended to produce a larger proportion of females. The
most minute quantitative measurements were made of this factor, to
eliminate any possibility of error.

The chromosome mechanisms practically force us to suppose that about
half the eggs are originally predisposed to maleness, half to
femaleness. A pigeon's clutch normally consists of two eggs, one with a
large yolk and one with a small yolk. But the half-and-half numerical
relation of males to females varies considerably--i.e., not all the
large-yolked eggs produce females or all the small-yolked ones males.

Wild pigeons begin the season by throwing a predominance of males, and
the first eggs of the clutches also tend to produce males all along. In
both cases, the male-producing eggs were found to be the ones with the
smaller yolks. Family crosses also produce small yolks, which hatch out
nearly all males. Some pairs of birds, however, have nearly all female
offspring. Riddle investigated a large number of these cases and found
the amount of yolk material to be large. In other words, there seems to
be a definite relation between the amount of yolk and sex.

A great number of clever experiments were carried out to find out if
eggs originally predisposed to one sex were actually used to produce the
other. Selective fertilization with different kinds of sperm was
impossible, since in these birds there is only one type of sperm--two of
eggs--as to the sex chromosome. For instance, by overworking females at
egg-production, the same birds which had been producing more males than
females were made to reverse that relation.

One of the interesting results of the experiments was the production of
a number of intersexual types of various grades. This was easily
verifiable by colour and other characteristics. To make sure that the
instincts were correspondingly modified, behaviour was registered on
moving-picture films. Where the first egg of a clutch (the one with a
small, normally male-producing yolk) produces a female, she is usually
found to be more masculine than her sister from the second egg with the
larger yolk. Th