Seasonal cyclic changes in the thymus gland of the adult frog, Rana temporaria.

The thymus gland of the adult common frog, Rana temporaria, undergoes conspicuous seasonal changes corresponding to the temperature-dependent changes in the annual activity of these animals. During the period of active terrestrial life the thymus is clearly divided into a lymphocyte-rich cortex and an epithelial medulla containing interdigitating-like cells. During hibernation and mating the thymus is depleted of lymphocytes and consists of numerous secretory units (cells and cysts) and degeneration cysts. Rapid release of secretory material is evident at the beginning of the frog active life.