Kenneth Walker’s[111] observation that the interstitial cells of the testis show a gradual diminution in number from the age of about 30 years, might be quoted in favour of the contention that atrophy of these cells plays a part in the production of senescence. In addition there are the dramatic experiments on rats carried out by Eugen Steinach,[112] who was formerly Professor of Physiology in the German University of Prague and since 1912 has been Director of the Physiological Section of the Experimental Biological Institute at Vienna. He found that in apathetic senile rats with degenerative changes in the thyroid, pituitary, and interstitial cells ligature or section of one vas deferens causes atrophy of the seminal tubes and active growth of the interstitial cells; the hormone thus provided stimulates the thyroid, pituitary, and brain, and the rat is rejuvenated, the sexual instincts and emotions become active, and the rat may have offspring; when relapse into senility occurs, a repetition of the operation on the other side brings about a further rejuvenation; and later the same result is effected by grafting the testes of a young rat. In this way life is definitely prolonged. This procedure has been advocated in man with some success by Steinach; Lichtenstern of Vienna, who has done 36 such operations and 21 implantations on account of senile changes, has not encountered any bad results, but admits that he has not been uniformly successful, and that the number of cases is not sufficient to justify a final conclusion. From observation in Vienna Benjamin[113] estimated that the operation was followed by no results at all in from 10 to 20 per cent of the cases. Obviously a considerable time must elapse before any decision as to prolongation of life can thus be assured. In the autumn of 1921 a man aged 70 was advertised to speak at the Albert Hall on “How I was made 20 years younger by the Method of Dr. Steinach of Vienna,” but he died on the morning of the day from pneumonia (Benjamin[113]), the necropsy not revealing, so it is said, any trace of an operation; the hypertrophy of the interstitial cells, however, may have been induced by the exposure of the testes to _x_-rays. Steinach’s observations naturally aroused great interest and have been repeated with discordant results by a number of investigators: Simmonds[114] and Tiedje[115] have failed to confirm the occurrence of the changes in the testes after vasectomy; and Romeis[116] and Marinesco[117] have not obtained the striking clinical results described by Steinach. Levy Lenz and Schmidt,[118] however, report rejuvenation in 23 out of 24 cases of vasectomy, and Voronoff encouraging results from implantation of chimpanzee’s testes into man. But Marinesco compares Steinach’s published results with those of Brown-Séquard.