In addition to these, other important differences between the sexes manifest themselves during the second period of childhood. In the first place, it is an established fact that in the girl the secondary sexual characters make their appearance earlier than in the boy, the boy remaining longer in the comparatively neutral condition of childhood. We have seen that in the girl, at the end of the first period of childhood, the lower half of the body begins to resemble that of the woman in type. During the second period of childhood, this peculiarity becomes more marked; the pelvis and the hips widen, the thighs and the buttocks become more and more rounded; the enduring feminine characteristics in these respects are acquired. More gradually, the feminine development of the upper half of the body succeeds that of the lower; the transition from the lower jaw to the neck become less abrupt, and the face becomes fuller. The sexual difference in the growth of the hair also manifests itself in childhood. Whether cut or uncut, the girl's hair tends to grow longer than the boy's. Later, the typical development of the breasts occurs. As early as the beginning of the second period of childhood, the surface of the areola mammae may become slightly raised; but the typical deposit of fat, leading to the hemispherical prominence of the breast, does not begin until towards the close of the second period of childhood. Even later than this is the growth of the axillary and pubic hair. Various answers are given to the question as to the relation in time between the appearance of menstruation and the development of the sexual characters just described. Unquestionably there are great differences in this respect. Whereas Axel Key declared that the secondary sexual characters appeared before the first menstruation, according to C. H. Stratz this is true only of girls belonging to the lower classes; whilst according to his own observations on girls belonging to the upper classes of society, the first menstruation precedes the development of the breasts and the growth of the pubic and axillary hair.