600 IODINE AND THE THYROID. Ill 



other environmental factors also play a part which may lead to 

 misinterpretation of results.^ 



In a recent number of this Journal, Uhlenhuth, while accepting the 

 conclusions stated regarding the relation of iodine to amphibian meta- 

 morphosis, thinks that still another substance is needed to cause the 

 thyroid gland to excrete the iodine necessary for metamorphosis. 

 This hypothetical factor he terms an excretor substance^ and thinks 

 that it is evolved during the growth processes of the organism. 



The assumption of an excretor substance obscures rather than 

 clarifies the already sufficiently complicated problem of amphibian 

 metamorphosis by the introduction of an unknown and purely hypo- 

 thetical agent. That iodine is essential for metamorphosis we have 

 seen, but it is equally certain that iodine is not the sole factor in- 

 volved, or else Anuran larvee of the different species, having different 

 peripds of larval life, should if reared together and fed equal amounts 

 of iodine undergo metamorphosis simultaneously. This they will 

 not do. The writer believes that the long larval life of Rana cates- 

 hiana is related to slow thyroid development in that species and the 

 consequent inability of the organism to utilize iodine except in large 

 amounts; conversely, that the short larval life of Bufo is due to very 

 rapid thyroid differentiation, hence greater powers of iodine assimila- 

 tion. There is no experimental proof to substantiate this view and 

 it is to be regarded merely as a suggestion. 



Amphibian metamorphosis is an extremely complicated phenomenon, 

 and in the last analysis will probably be found to depend, not on a 

 single factor, but on a group of factors both extrinsic and intrinsic, 

 the cooperation of which is necessary to complete the process. 



Growth. — The data on growth recorded for the iodine-fed tadpoles 

 showed that the size of the larvae varied considerably for each indi- 

 vidual culture; at a given time some of the animals would be 10 to 12 

 mm. longer than others of the same culture, but for the experiment 

 as a whole there was a continuous upward trend of the growth curve 



^ Gudernatsch, J. F., Feeding experiments with tadpoles. II, Am. J. Anat., 

 1913-14, XV, 431. Swingle, W. W., Experiments with feeding thymus glands to 

 frog larvae, Biol. Bull., 1917, xxxiii, 116. 



^ Uhlenhuth, E., Relation between thyroid gland, metamorphosis, and growth 

 /. Gen. Physiol., 1918-19, i, 473. 



