476 



THYROID, jVIETAMORPHOSIS, AND GROWTH 



the thyroid gland. Hence besides iodine still another substance is 

 needed in the amphibian metamorphosis; namely, the excretor sub- 

 stance which causes the thyroid to excrete the stored up iodine. 



It may be mentioned here that the sudden drop of the growth curve 

 at the time of metamorphosis may be explained if we assume that the 

 thyroid, when it is stimulated by the excretor substance, excretes at 



Fig. 1. 



first an overdose of iodine, while later the excretion becomes less 

 energetic. Janney^ has shown that while an overdose of thyroid 

 hormone leads to a negative nitrogen balance and a loss in weight and 

 size, a certain minimum dose results in a positive nitrogen balance 

 and a gain in weight and size. 



4 Janney, N. W., Arch. Int. Med., 1918, xxii, 187. 



