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The marsupial Didelphys virginiana (the North American opossumn) is uniquely suited for studies of mammalian limb replacement. By transplanting nervous tissute to the limb, regeneration has been successfully induced in this mammal.

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Injection of frog embryos (Rana pipiens) with a zonal centrifuge purified fraction of herpes-type virus (prepared from virus-containing Lucké tumors) resulted in a high incidence of kidney tumors. This partially purified oncogenic fraction contained a high concentration of an enveloped form of the frog herpes-type virus (adjacent fractions lacked this particle and were not oncogenic), which suggests that this form of the virus plays a role in the genesis of the Lucké tumor.

Spontaneous renal adenocarcinomas of Rana pipiens exist in two seasonal forms within the same populations of northern United States frogs. Tumors from animals collected in winter monlths display negligible mitotic activity and contain numerous cells characterized by enlarged nuclei with marginated chromatin and Cowdry (1) type A inclusions. A constant association of herpes-type virus with inclusion-conltaining cells has been demonstrated by electron microscopic studies of these naturally occurring winter tumors (2,3). In contrast, tumors from summer frogs are mitotically aotive and lack viral inclusioncontaining cells. These observations have led to the postulation of suggested life cycles for the Luck6 frog tumor (4) and for its associated virus ( 5 ) .This tumor dichotomy appears to be a temperature-related phenomenon: tumors arising in laboratory-housed frogs, regardless of season, display "summer" morphology when host animah are maintained at 2&26OC, or "winter" morphology when maintained at 5°C (6). The dependence of winter tumor m rphology upon low temperature was further substantiated by studies in which tumors with typical " s u m w " morphology developed "winter" characteristics (including intranuclear inclusions) after the tumor-bearing frogs had been maintained at 4OC for several months (7).In the present study "virus-free" primary tumors and eye chamber transplants from these tumors were subjected to prolonged low temperature treatment. Herpes-type virus developed both in primary tumors and in transplants, and herpesvirus was recovered from primary tumor.