144 E - UHLENHUTH. 



As Leo Loeb rightly points out, the only facts seriously in the 

 way of his own and Kendall's viewpoint are the experiments on 

 amphibians, especially those of Swingle, 3 which have shown that 

 administration of inorganic iodine causes precocious metamorphosis 

 of tadpoles. Loeb overcomes this obstacle by assuming that the 

 amphibian metamorphosis is not so much an indicator of thyroid 

 activity as of the action of iodine as such. Kendall 4 likewise sees 

 himself compelled to rule the amphibian metamorphosis out of the 

 phenomena caused by the specific action of the thyroid hormone, 

 and assumes now that thyroxin has a twofold effect, one on myx- 

 edema and basal metabolism, which is specific to the thyroid hor- 

 mone and dependent on the peculiar chemical constitution of it, 

 and one on the amphibian metamorphosis, which is merely due to 

 the iodine contained in the thyroid hormone and not specific for 

 the latter one, but for the inorganic iodine as such. 



In a previous article I have shown that the interpretation given 

 by myself and others to Swingle's experiments is not correct. 5 

 Although it may be true that the administration of inorganic iodine 

 enforces metamorphosis of the tadpoles, it must be kept in mind 

 that from the effect of the administration of a substance one can 

 not, with any certainty, conclude upon the effect of the adminis- 

 tered substance, unless the fate which this substance undergoes in 

 the body is known. As to the fate of the inorganic iodine in the 

 tadpole body, there is much evidence to show that it does not pro- 

 duce metamorphosis in the form of inorganic iodine, but after the 

 thyroid or in the absence of this gland other tissues similar in 

 their function to the thyroid gland has elaborated from it the 

 thyroid hormone. The results on iodine-fed tadpoles, therefore, 

 are neither new nor are they surprising as far as the ability of the 

 tadpole thyroid goes to produce more " active thyroid hormone," 

 if more iodine is administered ; it is long known that the mamma- 

 lian thyroid possesses exactly the same ability as shown by Marine 

 and Rogoff in quantitative experiments. 6 The only thing new in 

 Swingle's experiments is the discovery that the thyroid apparatus 



'Swingle, W. W., /. E.vper. ZooL, 1918-19, xxvii, 397-415. 



4 Kendall, E. C., Amer. J. Physiol., 1919, xlix, 136-137. 



5 Uhlenhuth, E., Endocrinology (in press). 



6 Marine, D., and Rogoff, J. M., /. Pharm. and E.vper. Ther., 1916, ix, i-io. 



