["Mixed" follicles in the rat thyroid gland (electron microscopic findings)].

The thyroid gland of the rats 3, 5, 7, 11 days and 1 month old has been studied by electron microscopy method. Mixed follicles with walls consisting of usual thyrocytes and unusual stratified epithelium have been observed. Particularly large cells containing glycogen are described in stratified zones. Structural relation of basal cells with marginal cells has been traced. Mitosis of basal cells has been demonstrated. The origin of mixed follicles is discussed and some doubts arise as to their ultimobranchial nature.