Development of the thyroid gland in the chick embryo

Abstract

Abstract 1. The developing chick thyroid consists of epithelial plates or cords suspended in a sinus, probably lymphoid. These cords in cross section have the appearance of follicles. 2. Growth in size is accomplished by fusion of follicles. Growth in number of the latter by proliferation of inter‐follicular epithelial cells. 3. Colloid first appears on the tenth day of incubation. The tadpole test indicates the beginning of functional activity at this time. 4. During development there is a progressive decrease in the cytoplasm‐nucleus ratio from 3.3: 1 on the third to 1.5: 1 on the nineteenth day, as measured in area, not volume. 5. Not only is chromophobe and chromophile colloid present in different cells of the same follicle at the same time, but both are occasionally present in the same cell at the same time. 6. Evidence of heightened activity in the chick thyroid are as Uhlenhuth (′28) describes them in salamanders: namely, high columnar cells, apically located secretion granules, and vacuoles of chromophobe colloid. 7. Follicles in late developmental stages have, at some time during their functional cycles, tubular exits, which are thought to represent one path through which colloid reaches the circulatory spaces. 8. Vital staining indicates that elements of the reticulo‐endothelial system are present in the developing thyroid. Phagocytic cells in the gland of one embryo were observed phagocytosing entire erythrocytes.