gical remains
of the latent sex, which always exist in the genital ducts."

In some lower forms, dual sexuality is apparent until the animal is
fairly well developed. In frogs, for example, the sex glands of both
sexes contain eggs in early life, and it is not possible to tell them
apart with certainty, until they are about four months old.[12, p.125.]
Then the eggs gradually disappear in the male.

However, we need not depend upon non-mammalian evidence for either the
secretory explanation or the dual basis. An ideal case would be to
observe the effects of circulating the blood of one sex in a developing
embryo of the other. This blood-transfusion occurs in nature in the
"Free-Martin" cattle.[21]

Two embryos (twins) begin to develop in separate membranes or chorions.
At an early stage in this development, however, the arteries and veins
of the two become connected, so that the blood of each may circulate
through the body of the other. "If both are males or both are females no
harm results from this...," since the chemical balance which determines
the bodily form in each case is of the same type. But if one is a male
and the other a female, the male secretory balance dominates the female
in a very peculiar fashion. The female reproductive system is largely
suppressed. She even develops certain male organs, and her general
bodily appearance is so decidedly masculine that until Dr Lillie worked
out the case she had always been supposed to be a non-functional male.
She is sterile. The blood transfusion not only alters the sex-type of
her body, but it actually modifies the sex glands themselves, so that
the ovary resembles a testicle, though dissection proves the contrary.

Why does not the female become a true, functional male? Perhaps she does
in some cases. Such a one would not be investigated, since there would
be no visible peculiarity. In all the cases examined, the embryo had
begun its female development and specialization under the influence of
a predisposition of the female type in the fertilized egg, before the
transfusion began. There is no absolutely convincing mammalian evidence
of the complete upset of this predisposition, so all one can say is that
it is theoretically possible. Cases of partial reversal, sometimes
called "intersexes," are common enough. In birds and insects, where the
material is less expensive and experimentation simpler, males have been
produced from female-predisposed fertilized eggs a