In addition to the seminal tubules and ova, which provide their external secretion, the testes and ovaries contain the interstitial cells described by Bouin and Ancel[101] or, in Steinach’s phrase, “the puberty gland”; the germ cells and the interstitial cells differ from each other in structure, function, and reaction to external influences; thus the germ cells though dominant are more susceptible to damage, for example by ligature of the vas deferens, _x_-rays, alcoholism, or pressure, as in the case of a cryptorchid. It is stated that when the seminal tubules are active and prominent the interstitial cells are scanty, and conversely that when the seminal tubules atrophy the interstitial cells multiply; Lipschülz, Ottow, Wagner, and Bormann[102] showed that hypertrophy of the interstitial cells as observed in various experimental conditions depends on local factors in the testes and has nothing to do with their internal secretion in relation to the body as a whole. After ligature of the vas deferens in animals there are, according to Steinach, Kuntz,[103] and others, two stages (1) the seminal tubules atrophy while the interstitial cells multiply, and (2) after some time the seminal tubules regenerate while a certain amount of increase in the interstitial cells persists. Nathan[104] ascribes the regeneration of the seminal tubes to the influence of the interstitial cells, which he considers act like young connective tissue by favouring growth. The interstitial cells of Leydig in the testis and the interstitial or lutein cells of the ovary are generally regarded as responsible by their lipoid internal secretion or hormone for the secondary sex characteristics (Bouin and Ancel, K. Sand[105]), though Blair Bell[106] and Sternberg[107] deny this. Sir Frederick Mott,[108] who has extensively investigated the interstitial cells, especially in connexion with dementia praecox and other forms of mental degeneration, concludes that in fetal life they act as sex determinants, and that later in life they are responsible for the sexual appetite. It has been argued that atrophy of the interstitial cells is the cause of senescence. Against this view is Sir Frederick Mott’s observation that after birth the interstitial cells undergo progressive atrophy and disappear, that they reappear and are in a state of functional activity at puberty (a sequence of events described by Aron[109] as two different interstitial glands), and that they are present in extreme old age; Retterer[110] also described their presence in the testes of men over 70 years of age. So far it might be concluded that changes in old age, such as diminution in number and pigmentation, shown by the interstitial cells, are not the cause but an accompaniment of senescence.