Of the half-dozen or more expert opinions upon Steinach’s work nearly all have been by his own countrymen and by far the most exhaustive and, on the whole, highly favorable is that of Paul Kammerer.[185] For a very condensed account of it in English see A. Granet’s résumé[186] in which he says (1) that Steinach’s work is based on a new conception of the puberty gland as the internal secretory portion of the gonads. This consists of the interstitial cells in the male and of the lutein cells in the female. (2) Steinach began by studying animals with a protracted rutting period in alternating stages of development of the interstitial gland and the generative gland proper. He found a periodical hyperdevelopment in the evolution of every individual, the interstitial gland predominating in infancy and attaining its maximum development at puberty and adolescence, when growth and vital energy are also at their maximum. At this time the generative gland increases and both the interstitial and generative portions continue to be about equally active till the climacteric, after which there is rapid recession of the interstitial gland, and this causes senility, which is not due to the ultimate using up of all elements but to the lack of potential stimulus due to degeneration of the interstitial gland. (3) Steinach used this alternating balance of nature in the mixed gland by artificially inhibiting the generative portion and thereby causing compensatory regulation and revival of the interstitial portion with all its rejuvenating effects and the recession of the traits of senility. This he accomplished by three methods (_a_) simple ligation, under local anæsthesia, of the vas deferens. This causes regression of the generative gland and a compensatory regeneration of the interstitial portions. A one-sided operation is sufficient in all cases and has the advantage of preserving in addition the power of procreation. Of course ligation of the Fallopian tube in the female does not produce this result. (_b_) Repeated mild exposure of the gonads to the X-ray is a slower but apparently just as effective a means of obtaining the same result for both ovaries and testes. And lastly, (_c_) the effects of rejuvenation may be experimentally produced by transplantation in the old of the respective gonads of the young animal of the same species. For years Steinach bred and reared healthy generations of laboratory animals and studied their dispositions, habits, physical and psychic traits, until he has become unprecedentedly expert in diagnosing age, to say nothing of sex. The increased resistance to disease and the actual prolongation of life of the operated animals he estimates at about 25 per cent but after a time senescence sets in again.