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;