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. 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 which should theoretically have been the most feminine were dissected and shown to be so. That is, out-crosses which produced a predominance of females in the fall were mated with females which had been overworked at egg production until they threw nearly all females. Dissecting the females thus produced, they were shown to have _right ovaries_, which means _double femaleness_, since normally the pigeon is functional only in the left ovary, like other birds. The right one usually degenerates before or at hatching and is wholly absent in the week-old squab.