The early development of the thyroid gland in the dog is of especial interest because of the precocious appearance of branching cell cords from the thyroid plate. This peculiar mode of development leads naturally into two main lines of investigation which have been made the subject of this paper. The first of these was to determine what light this peculiar manner of development might throw upon the interpretation of the thyroid as an endocrine gland. The second was to determine the manner of origin of accessory thyroid tissue within the thorax especially that upon the aortic arch.The thyroid gland of Petromyzon has been shown by Schneider (1879) to arise from the endostyle organ during the metamorphosis of the larval Ammocoetes. Marine (’13 a ) has shown that one or possibly two of the five types of epithelial cells of the endostyle organ take part in the forma- tion of thyroid follicle: The other cell types degenerate. Dohrn (1886) found pharyngeal grooves in early development of rays and sharks which he believed to be remnants of the endostyle organ. Studies of fishes, amphibians, reptiles, and birds reveal no endostyle organ but show that the thyroid develops as a median downgrowth of the pharyngeal floor.