We have already shown in rude outline how a difference (apparently 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 stature, and _vice versa_. What is commonly known as menstruation lasts only a few days, and is merely the critical period in a monthly cycle or periodicity which goes with the female sex specialization. This period involves the gradual preparation of the uterus or womb for a guest, together with the maturing of the ova. Then the Graafian follicles containing the ova break and these latter enter the uterus for fertilization.