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  When a single chemical element (the hormones from the sex-glands) was withdrawn, the system (thus stereotyped in a developed body and glands) was modified but not entirely upset.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;

  b.  Increasingly stronger as the new body becomes organized and
  developed;

  c.  Liable to partial or complete upset in the very early stages;

  d.  Probably quantitative—­stronger in some cases than in others.

The new definition is, then, really a combination and amplification of the three older points of view.

The term “sex determination” does not mean to the biologist the changing or determining of the sex at will on the part of the experimenter.  This might be done by what is known as “selective fertilization” artificially with only the kind of sperm (X or Y as to chromosomes) which would produce the desired result.  There is as yet no way to thus select the sperm of higher animals.  It has been authoritatively claimed that feeding with certain chemicals, and other methods to be discussed later, has affected the sex of offspring.  These experiments (and controversies) need not detain us, since they are not applicable to the human species.