chemical) between two fertilized eggs starts them along two different
lines of development in the embryonic stage. One develops the
characteristic male primary and secondary sex characters, the other the
female. Throughout the embryonic or intra-maternal stage this
differentiation goes on, becoming more and more fixed as it expresses
itself in physical structures. Childhood is only a continuation of this
development--physically separate from the mother after the period of
lactation. Until puberty, when sex ceases to be merely potential and
becomes functional (about 12-14 in girls and 14-16 boys), the
differences in metabolism are not very marked. Neither are they in old
age, after sex has ceased to be functional. It is during the period when
sex is functional (about 35 years in women and considerably longer in
men) that the gross physiological differences manifest themselves.

Before puberty in both sexes, calcium or lime salts are retained in the
tissues and go to build up the bony skeleton. (A mere sketch of calcium
metabolism is all that can be given here--for details consult such works
as 15 and 17 in bibliography; summary in 14; pp. 34f. & 161f.) Note that
puberty comes earlier in girls than in boys, and that the skeleton
therefore remains lighter. During the reproductive period in women these
salts are heavily drawn upon for the use of the reproductive system. The
male reproductive system draws upon them as well, though the drain is
very slight as compared to that in women. In old age these salts produce
senility through deposit in the tissues, especially in the arteries.

At the pubertal age in girls begins the phenomenon known as
menstruation, in which there is a large excretion of calcium salts. In
pregnancy these are needed for building the skeleton of the foetus, and
at delivery go to the breasts to assist in lactation. Bell states that
there is a noticeable connection between early menstruation and short