This threefold evidence demonstrates that in the male of the mammalia most, perhaps all, of the secondary sexual characters that are affected by castration are not affected by the destruction of the germ-cells. This conclusion supports very strongly the view that the interstitial cells are the cellular element in the testes that influence through internal secretion the development of the secondary sexual characters of the male. Equally important are the results that relate to the accessory organs of reproduction, such as the glands that open into the vas deferens (prostate, Cowper’s gland, etc.) and the copulatory organs also. In the castrated mammals these organs diminish in size. On the other hand, after destruction of the germ-cells in the testes (or even when they fail to develop as in cryptorchid individuals) these accessory parts are unaffected. In birds, as will be shown, the situation is entirely different. CYCLICAL CHANGES IN THE INTERSTITIAL CELLS IN HIBERNATING MAMMALS. The changes that take place in the interstitial cells in mammals that hibernate and in which there is a definite rutting season following hibernation have been examined by several workers. The mole has been studied by Regaud (1904), Lécaillon (1909), Tandler and Grosz (1911); the marmot by Hauseman (1895) and Gaugini (1903); the hedgehog by Marshall (1911); and the woodchuck by Rasmussan (1917). In the mole the interstitial cells are most abundant when the tubules in which the spermatogenesis is taking place are least developed, and _vice versa_. In the hedgehog the increase in both tissues takes place at the same time. In the woodchuck both tissues increase rapidly after hibernation (during March and April), after which the spermatogenesis continues actively for the two following months (May and June), while the interstitial cells retrograde rapidly during April and remain at a low level for the rest of the year. Retrogression in the germinal epithelium begins in July, after the rutting season is past. It appears from this evidence that the activity of the two tissues does not always run the same course. Since the secondary sexual characters of the male, which are not well developed in these animals, are not so far as known affected by the condition of the testes, the evidence does not have any very direct bearing on our present topic. How far the sexual behavior of these mammals is determined by the quantity or by the activity of the interstitial cells is not very clear from the evidence, although there is a very noticeable increase in the amount of this tissue just before and during the rutting season. In the mole also the interstitial cells begin to increase just before the mating season, and the increase continues for several months after mating has taken place. It is difficult to judge how great or how little the change amounts to unless the whole organ is considered, for the relative volumes of the seminal