Many investigators have found that the Röntgen rays destroy the spermatic cells of the testis in Mammals, leaving the cells of Sertoli, the interstitial tissue, nerves, and vessels uninjured. Tandler and Gross [Footnote: _Wiener klinische Wochenschrift_, 1907.] found that the antlers of roebuck were not affected after the testes had been submitted to the action of the rays, showing that the interstitial cells were sufficient to maintain the normal condition of the antlers. Simmonds, [Footnote: _Fortschr. a. d. G. d. Röntgenstr._, xiv., 1909-10.] however, found that isolated seminal tubules remained, and regeneration took place, and concludes that both spermatic cells and interstitial cells take part in producing the testis hormone. The conclusions of two other investigators have an important bearing on this question--namely, that of Miss Boring [Footnote: _Biol. Bull._, xxiii. 1912.] that there is no interstitial tissue in the bird's testis, and that of Miss Lane-Claypon, [Footnote: _Proc. Roy. Soc._, 1905] that the interstitial cells of the ovary arise from the germinal epithelium, and are perfectly equipotential with those which form the ova and Graafian follicles. It seems possible, although no such suggestion has been made, that the interstitial cells might either normally or exceptionally give rise to ova and spermatocytes. The observations of Seligmann and Shattock on the relation of spermatogenesis to the development of nuptial plumage in drakes probably receive their explanation from the above facts. Spermatogenesis is not the only source of the testicular hormone: changes in the secretory activity of the interstitial cells or spermatocytes are sufficient to account for periodic development of somatic sex-characters, and the same reasoning applies to the antlers of stags. THE MAMMARY OR MILK GLANDS