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. This is true both as to appearance and as to behaviour.
Some of these were quite nearly males in appearance and behaviour,
though they laid eggs.

Testicular and ovarian extracts were injected. The more feminine birds
were often killed by the testicular extract, the more masculine by the
ovarian extract. Finally, to make assurance doubly sure, some females