In another experiment the rabbit experimented upon bore young when it was six and a half months old. Soon after the birth of the young and before the mamma had been used the animal was killed and the single mamma that had been left was measured. It was much enlarged and projected more than the normal mammæ. It measured nine by five centimetres. In a normal control animal[53] the corresponding mamma measured seven by five centimetres. The number of acini was in the proportion of sixteen in the animal operated upon to ten in the normal. The results show a distinct compensating hypertrophy, due to a hyperplastic increase in the number of elements of the gland. A further example of compensating hypertrophy has been found after the removal of the spleen, when the lymphatic glands of other parts of the body become enlarged. There are also observations which go to show that after the removal of some of the lymphatic glands others undergo an enlargement. Ziegler[54] has given a critical review of the various opinions and hypotheses that have been advanced to account for the process of hypertrophy. According to Cohnheim[55] hypertrophy in bones, muscles, spleen, and glands is due to hyperæmia, _i.e._ increased blood supply. He thinks that neither mechanical nor chemical stimuli can cause directly new processes of growth. Recklinghausen[56] thinks that hypertrophy is not due to any extent to an increase in the food supply. Samuel[57] explains hypertrophy as due to a removal of, or to a decrease in, the resistance to growth and also to the influence of the nerves. Klebs[58] thinks that three factors enter into the problem, (_a_) inherited peculiarities, (_b_) overfeeding, (_c_) a removal of the controlling influences. Weigert believes that reparative processes are due to the removal of influences that prevent growth, and not to a direct stimulus. He thinks that a stimulus may start a functional act, but can never start a nutritive or a formative one. Good nourishment, for instance, may bring a tissue to a maximum development that is predetermined by innate peculiarities, but “idioplastic forces” are not thereby increased. Pekelharing[59] thinks that hypertrophy is due to a disappearance of a resistance to growth, and also to a stimulus causing proliferation. We see from these various opinions how little is really known; how little has been determined as yet by experiment as to the causes that bring about hypertrophy. Many of the views are more or less plausible in the absence of direct, experimental evidence, but it remains for the future to decide as to the correctness of all of them. They are valuable as suggestions, in so far as they show the different possibilities that must be taken into account.