Naturally occurring amphibian tumors are subject to geographic and seasonal fluctuations. In the frog, Rana pipiens, Lucke renal adenocarcinoma has been reported to occur in spring and autumn, but not in summer.f>> Moreover, the incidence of this tumor is restricted to the frogs from certain areas of Minnesota (U.S.A.) J Differences in geographical or local abundancy of papilloma or epithelioma in the Japanese newt, Cynops pyrrhogaster, have also been reported.` '4 The field prevalence of newt epitheliomas was high in autumn, but much lower in the rest of the year. These amphibian tumors were found to be associated with the presence of a One of the causative factors involved in the seasonal and geographical differences in tumor frequency in the field is temperature.4> This paper deals with the early morphological changes of epithelioma cells in the newt in response to experimental temperature conditions in the laboratory. Materials and methods. Newts papillomas and temperature conditions : Of about 6500 specimens of the Japanese newts, Cynops pyrrhogaster, collected from the fields, ponds, and small creeks in Niigata and Iwate Prefectures in the autumn of 1982 and 1983, 271 newts (:4.2%) with papillomas were found. These were separated from the rest and kept in the laboratory aquarium for 2 months at 13°C. One-hundred and forty-four of the newts bearing tumors of 2.0-2.5 mm in diameter were divided into 4 groups of 36 each, and were exposed to different constant temperatures, 4°C (Group 1), 13°C (Group 2) and 30°C (Group 3), respectively. Group 4 (control) was kept under natural fluctuating temperature conditions. The newts were fed with freshwater oligochaetes (Tubif ex hattai) throughout the experiments) and papilloma diameters were measured weekly in all of them. Histological studies : Papillomas from 4 newts were fixed in Bouin's solution at 1, 2, 4, 8, 16 and 30 weeks, embedded in paraffin, sectioned at 81 m and stained with Delafield's hematoxylin and eosin. The mitotic indices of papilloma cells were estimated on the basis of at least 1000 nuclei counted in each section. Results. The normal skin of the control newts consisted of three cell layers. The surface epidermal layer was 2 to 3 cells thick. At the bottom of the epidermis, there was a concentrated melanin pigment layer under which the dermis was situated. J°C: After 8-week exposure to 4°C, epitheliomas appeared to be regressive (Fig. 1). A large number of pycnotic or condensed nuclei occurred between the epidermis and dermis (Fig. 2a). In some histological sections, down-growths
