ELECTRON MICROSCOPE OBSERVATIONS ON INTRACELLULAR VIRUS-LIKE PARTICLES ASSOCIATED W ITH T H E CELLS OF T H E LUCKt~ RENAL ADENOCARCINOMA* BY DON W. FAWCETT, M.D

Relatively few malignant neoplasms have been described in amphibia. The one that has been studied most extensively is the Luck6 renal adenocarcinoma of the frog, Rana pipiens. This malignant tumor occurs in from two to four per cent of frogs of this species, collected in the Lake Champlain region of northern Vermont and adjoining areas of Quebec. In a series of papers Luck6 described its histopathology (17), reported the occurrence of distant metastases (18) and demonstrated the presence in some of the tumors of intranuclear inclusion bodies of a kind commonly seen in virus diseases (17, 22). I t was subsequently shown by him that tumors could be induced in frogs of the susceptible race by injection of desiccated or glycerinated preparations of tumor. Similar inoculation of other races or species was without effect. These observations led Luck6 to conclude that the tumors were probably caused by a species-specific, organspecific agent having the characteristics of a virus (19, 20). Additional supporting evidence for this conclusion was provided in later work of Schlulnberger and Luck6 (31) in which the tumor was carried for many generations by serial intraocular transplantation. In the course of these experiments it was noted that frogs bearing intraocular tumors very often developed kidney tumors. Similar results have been reported by Rose and Rose (30) in anterior chamber transplants in adult frogs, and by Briggs (4) in tadpoles with tumor implants in the tail fin. In both instances persistently growing adenocarcinomas frequently developed in the kidneys even though the implanted tumor tissue itself regressed. The most reasonable interpretation of these findings seems to be that a tumor agent passed from the implanted tissue to the kidney and there induced a tumor of the same kind. Further evidence of the viral nature of the tumor-inducing material has been provided by the work of Duryee and Doherty (8, 9) who have recently established its filtrability. In the present investigation, thin sections of frog renal tumors have been