RESUMEN
Thyroid glands from 80 human fetuses, 7～31 weeks in gestational age were obtained to study the morphological development of thyroid glands by histochemical, immunohistochemical and TEM techniques. During 7～8.5 weeks the glands composed mostly of epithelial cell cords and abundant glycogen were seen in the cytoplasm of the cells. Some follicles and small accumulations of colloid in the center of the follicles appeared at 12～12.5 weeks and less glycogen was found in the cells. C cells were distributed mainly in the posterior region of the upper pole of both lobes, they usaually occurred singly or in small groups. During intrauterine life, C cells occupied three positions. They are found between follicular cells, between follicular cell and basement membrane and between follicles. But they are situated predominantly in the follicular wall. Three morphological variants were found in the human fetal C cells: (1) spherical or oval cells, (2) polygonal or pyramidal cells (3) cells with a cytoplasmic process. Developing C cells seem to have few secretory granules by TEM and argyrophilic reaction in semithin sections.