Experiments of the sort just described are most easily carried out on the paired organs of the body, such as the salivary glands, the tear glands, the mammæ of the female, and the testes of the male. In regard to the latter two organs the evidence, especially in the case of the testes, is conflicting, but the recent experiments of Ribbert seem to give definite results. Nothnagel had found that after the removal of one testis there is no hypertrophy of the other. He pointed out that this result does not stand in contradiction to his hypothesis in regard to the kidneys, for the loss of one testis does not lead to a greater functional activity in the other. Each acts for itself alone. The result shows further, he adds, that the process of hypertrophy is not an adaptive one, but a physical or a physiological process. Ribbert on the contrary thinks that even Nothnagel’s statistics give evidence of hypertrophy, and Ribbert’s own experiments give unmistakable evidence of a considerable enlargement of the remaining testis. In his experiments, young rabbits were used that were born of the same mother and in the same litter. One of the testes was removed from some of the individuals, and after some months the remaining testis was taken out and its weight compared with that of the control animal. In sixteen out of seventeen experiments there was found to be a noticeable increase in the single testis as compared with either testis of the control animal. The results show that in some cases the single testis weighs almost as much as the two together of the control animal. It is important also to notice that in this case the enlargement has taken place in an organ that has not been active, as was the case with the kidney. Ribbert has also shown that hypertrophy takes place in the mammæ of the rabbit after the removal of some of them. Five out of the eight mammæ were removed in three cases, and seven out of the eight in two other cases from young rabbits about two months old. Ribbert found that if the operator is not careful to remove completely all the tissue of a mamma an active regenerative process takes place from the part that remains. After five and a half months the single remaining mamma of one animal measured six and one-half by three and four-fifths centimetres, and the corresponding one in the control animal five and three-fourths by three and one-half centimetres. The glandular tissue was also found less developed in the control animal.