The bodily peculiarities of each sex, as distinguished from the sex-glands or gonads themselves, are known as secondary sex characters. To put our statement in the paragraph above in another form, the primary and secondary sex do not always correspond in all details. We shall find as we proceed that our original tentative definition of sex as the ability to produce in the one case sperm, in the other eggs, is sometimes difficult to apply. What shall we say of a sterile individual, which produces neither? The problem is especially embarrassing when the primary and secondary sex do not correspond, as is sometimes the case.
Even in a fully grown animal, to remove or exchange the sex glands (by surgery) modifies the bodily type. One of the most familiar cases of removal is the gelding or desexed horse. His appearance and disposition are different from the stallion, especially if the operation takes place while he is very young. The reason he resembles a normal male in many respects is simply that sexuality in such highly-organized mammals is of 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 When a single chemical element (the hormones from the sex-glands) was withdrawn, the system (thus stereotyped in a developed body and glands) was modified but not entirely upset.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.
For a long time a controversy raged as to whether sex is determined at the time of fertilization, before or after. Biologists now generally prefer to say that a fertilized egg is "predisposed" to maleness or femaleness, instead of "determined." The word "determined" suggests finality, whereas the embryo appears to have in the beginning only a strong tendency or predisposition toward one sex type or the other. It is now quite commonly believed that this predisposition arises from the quantity rather than the quality or kind of factors in the chemical impetus in the nuclei of the conjugating gametes. A later chapter will be devoted to explaining the quantitative theory of sex.
Hence the modern theory of "sex determination" has become:
1. That the chemical factors which give rise to one sex or the other are present in the sperm and ovum before fertilization;
2. That a tendency or predisposition toward maleness or femaleness arises at the time of fertilization, depending upon which type of sperm unites with the uniform type of egg (in some species the sperm is uniform while the egg varies);
3. That this predisposition is:
a. Weaker at first, before it builds up much of a body and gland system to fix it;