The power of growth is possessed by every living thing, but growth is not limited to the living.  Crystals also will grow, and the rapidity and character of growth and the maximum size of the crystal depends upon the character of the substance which forms the crystal.  From the single cell or ovum formed by the union of the male and female sexual cells, growth is continuous until a size corresponding to the type of the species is attained.  From this time onward growth is limited to the degree necessary to supply the constant loss of material which the body undergoes.  The rapidity of the growth of the body and of its component parts differs at different ages, and becomes progressively less active from its beginning in the ovum until the adult type of the species is attained.  As determined by the volume, the embryo increases more than ten thousand times in size during the first month of intra-uterine life.  At birth the average weight is six and a half pounds; at the end of the first year eighteen and a half pounds, a gain of twelve pounds; at the end of the second year twenty-three pounds, a gain of four and a half pounds.  The growth is cooerdinated, the size of the single organs bearing a definite ratio, which varies within slight limits, to the size of the body, a large individual having organs of corresponding size.  Knowing that the capacity of growth is one of the inherent properties of living matter, it is much easier to understand the continuance of growth than its cessation.  It is impossible to avoid the conclusion that there is some internal mechanism of the body which controls and regulates growth.  In the first chapter reference was made to organs producing substances which pass directly into the circulation; these substances act by control of the activities of other parts, stimulating or depressing or altering their function.  Two of these glands, the thymus, lying in front, where the neck joins the body and which attains its greatest size at puberty, and the pituitary body, placed beneath the brain but forming no part of it, have been shown by recent investigations to have a very definite relation to growth, especially the growth of the skeleton.  The growth energy chiefly resides in the skeleton, and if the growing animal has a diet sufficient only to maintain the body weight, the skeleton will continue to grow at the expense of the other tissues, literally living upon the rest of the body.  Disease of the glands mentioned leading to an increase or diminution or alteration of their function may not only inhibit or unduly increase the growth of the skeleton, but may also interfere with the sexual development which accompanies the skeleton growth.