Although excellent studies have been made 011 the morphogenesis and early function of the thyroid gland of the chick embryo, there is still disagreement as to the exact time that the gland assumes a functional role. I n these studies, although different techniques have been employed, all have been based upon the fact that a correlation exists between the storage of iodine-containing colloid and the functional activity of the thyroid.
Bradway ( '29) has reported that cliromophilic colloid is stored in the thyroid follicles on the 12th day of incubation and that discrete follicles appear for the first time on the llt,h day. She assumes that if the storage of chromophilic colloid is considered the morphological criterion of functional activity, then the 12th day is the earliest stage of development in which the thyroid gland becomes functional.
Hopkins in 1935, however, reported that chromophilic colloid is first formed as minute droplets on the 10th day and from this he was able to induce metamorphosis in tadpoles. His observations are in agreement with those of Sun ( ' 3 2 ) , p7ho using a biochemical approach, obtained a positive test for iodine in the embryo on the 10th day of incubation.
Gorbman and Evans ('41), using radioactive iodine, demonstrated in the tadpoles of the frog, Hyla regilla, that thyroid function begins very early after the formation of discrete