the whole body, not of the sex-glands or organs alone.

Suppose this horse was desexed at two years old. Nearly three years had
elapsed since he was a fertilized egg. During the eleven months or so he
spent within his mother, he developed a very complicated body. Beginning
as a male, with a male-type metabolism (that is, as the result of a
union between an X and a Y chromosome, not two X's), all his glands, as
well as the body structures they control, developed in its presence. Not
only the sex glands, but the liver, suprarenals, thyroid--the whole body
in fact--became adjusted to the male type. He had long before birth what
we call a male sex complex. Complex it is, but it is, nevertheless, easy
enough to imagine its nature for illustrative purposes. It is simply all
the endocrine or hormone-producing organs organized into a balanced
chemical system--adjusted to each other.

When the horse had had this body and this gland system for nearly three
years (eleven months within his mother's body and twenty-four outside),
it had become pretty well organised and fixed. When a single chemical
element (the hormones from the sex-glands) was withdrawn, the system
(thus stereotyped in a developed body and glands) was modified but not
entirely upset. The sex complex remained male in many respects. It had
come to depend upon the other chemical plants, so to speak, quite as
much as upon the sex glands. The later the castration is performed--the
more fixed the body and gland type has become--the closer the horse will
resemble a normal male. Much laboratory experimentation now goes to
show that some accident while this horse was still a fertilized egg or
a very small embryo might have upset this male type of body
chemistry--perhaps even caused him to develop into a female instead, if
it took place early enough. This is well illustrated by the so-called
"Free-Martin" cattle, to be described later.