In Steinach’s experiments with rats, which pass through the life stages so rapidly, he used the method of transplantation of testicular glands furnished by three-months-old individuals and this grafting need not necessarily be _in situ_ but in various parts of the body. If intussusception took place, as it generally did if the operation was well performed, the change here was generally marked within two weeks, as his photographs show. More or less of Steinach’s work has been confirmed, Ebstein tells us, by other observers who have shown that not only in rats but in guinea pigs the transfer of ovaries and testes between the sexes makes the male, to some extent, become female, and _vice versa_. Sex differences, Steinach thinks, do not result from anatomical differences in the organs transferred but are due to functions residing in certain cells, especially those of Leydig or Lutein. It is their secretions that determine sex characteristics. Indeed, they are really glands and vitality and vigor depend upon their state. Youth is the freshening up of these glands. No one has recognized more clearly than Steinach that there is a false old age that has been, in a sense, imposed by civilization upon elderly people and given them a rôle they have more or less passively accepted, just as in the same way there are spurious forms of other diseases. Some of Steinach’s critics have suggested that all he has done is to throw off these artificial inhibitions and give old age the true character nature intended it to have. But even if this criticism has any weight against his conclusions respecting old age in man, it certainly cannot apply to his studies of senescent animals, for in them the traits of old age were unmistakable, as not only photographs but, far more, activities showed. They certainly do seem to be really rejuvenated and not merely to be laying aside a sham old age.