Endogenous rhythm in the thymus gland of Rana temporaria (morphological study).

In European common frogs, Rana temporaria, leading an active land life at high external temperatures (May-October), the thymus glands are clearly divided into lymphocyte-rich cortex and mainly epithelial medulla ('summer' appearance). In contrast, thymuses of hibernating and mating frogs (at low temperatures, October-April) are typically endocrine organs with an abundant population of secretory cells, small number of thymocytes and lack cortico-medullary division ('winter' appearance). Thymuses of experimental animals kept from September until January at a high temperature tend to change their appearance from the 'summer' type to the 'winter' one. Thymuses of experimental frogs kept from the end of May until the end of June in refrigerator preserve their 'summer' appearance. These results show that the annual rhythm of the frog thymus gland is difficult to alter by simple changes of external temperature.