(1) The cyclical changes in the testes and thumb of male frogs are described. It is found that after the breeding season, although the outer papillated layers of the thumbpads are thrown off, yet the pads do not immediately become smooth, papillas of considerable size being present in the early summer months. The assumption of the perfectly smooth condition takes place gradually during the summer, and is due, not to a process of reduction, but to a proliferation of the epidermis which fills in the valleys between the papillae.
(2) Transplantation of testes into other individuals, whether male or female (allo-transplantation), leads to the breaking-up and degeneration of the ripe spermatozoa and the testicular tissue, and its replacement by fibrous tissue, the greater part of which is derived from the host and invades the degenerate testis from the place of attachment to the host's body. Testes transplanted into the peritoneal cavity of another individual always become attached to the peritoneal lining and rapidly vascularised. Phagocytosis plays an important part in the absorption of the degenerate spermatozoa.
(3) When the testes are simply severed from their connections in the body and left unattached in the peritoneal cavity of the same individual (auto-transplantation), they soon acquire new attachments and vascularisation. The ripe spermatozoa rapidly degenerate and are finally replaced by fibrous tissue, partly, at any rate, derived from the testis itself, but the spermatogonia of the peripheral tubules survive, and proliferate in an active manner. This survival of the germinal cells in the case of auto-transplantation never occurs in allo-transplantation.