162 IODINE AND THE THYROID. IV 



If this proved to be true, then the minimum figure should be much 

 lower for animals with thyroid glands than for those without. 



A brief resume of the facts brought to light in earher papers on this 

 subject are: (1) Inorganic iodine and its compounds, iodoform and 

 potassium iodide, greatly accelerate metamorphosis of tadpoles. (2) 

 Anuran larvae from which the thyroid gland had been removed at its 

 inception {i.e. 6 mm. larvae) and which under normal conditions do not 

 undergo metamorphosis, but grow to abnormal size, quickly trans- 

 form into frogs when fed elemental iodine. This indicated that iodine 

 is essential for amphibian metamorphosis, that it is the active constit- 

 uent of the thyroid glands of these animals, and, judging by its physi- 

 ological effect on thyroidless tadpoles, that it exerts its activity di- 

 rectly upon the cells and tissues of the organisms; i.e., that iodine is 

 capable of functioning like the thyroid hormone in thyroidless tad- 

 poles, and the thyroid-Hke action is either inherent in the iodine atom, 

 or else the elemental iodine is transformed into a substance similar 

 to thyroid secretion by tissues other than those which normally per- 

 form this function. (3) The thyroid follicles of tadpoles on an iodine 

 diet show a greater colloid content than do the glands of normally fed 

 animals. (4) The pecuhar action of iodine in accelerating meta- 

 morphosis is apparently unique and not possessed by other c'osely 

 related chemical agents like bromine. (5) Anuran metamorphosis 

 depends upon the amount of iodine secured by the larvae ; the greater 

 the quantity the more rapid the differentiation. 



Materials and Observations. 



The experimental animals were tadpoles of the common toad, 

 Bufo lentiginosus; all came from the same egg mass and hence pre- 

 sumably were of the same parentage. The thyroid anlagen of 100 

 animals were extirpated at the time of their evagination from the 

 pharyngeal floor, and the larvae appropriately controlled.^ When the 

 larvae were 9 mm. long, control and thyroidless groups were divided 

 into five cultures each, twenty larvae to a culture. The containers 

 were large, glass bowls containing 3,000 cc. of tap water. Four 

 thyroidless cultures and four cultures of normal animals were used 

 for experimental purposes; the remaining two cultures served as 



2 The writer is indebted to Miss M. E. Larson for the larvae. 



