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.

Let us consider this fertilized egg--the contributions of the father and
the mother. The total length of the spermatozoon is only about 1/300 of
an inch, and 4/5 of this is the tail. This tail does not enter the