IODINE AND AMPHIBIAN METAMORPHOSIS. 243 



organically combined in a certain way. The type of combina- 

 tion is not necessarily that characteristic of thyroid iodine 

 because a large number of iodine compounds have the power to 

 bring on metamorphosis in thyroidless amphibians even ele- 

 mental iodine itself. That elemental iodine is utilized within 

 the organism as iodine per se for any purpose seems improbable 

 because if one compares the effect of administering inorganic 

 iodine and various organic preparations to thyroidless tadpoles, 

 it becomes evident that the physiologic activity and metamor- 

 phosis-inducing properties of the organic preparations are superior 

 to elemental iodine. Furthemore, the accelerating effect upon 

 metamorphosis of the organic iodine compounds is less than that 

 of the thyroid extract itself. However, the important thing is 

 not the speed with which iodine compounds induce metamor- 

 phosis in comparison with the thyroid hormone itself, but the 

 fact that iodine other than thyroid iodine induces the metamor- 

 phosis of thyroidless animals. The crux of the problem in regard 

 to amphibian metamorphosis, is to find out just what it is in 

 tyrosine, serumalbumen, serumglobulin, casein, tyramine and 

 probably a host of other amino acids and proteins which w^hen 

 iodine is added increases so remarkably the metamorphosis- 

 inducing powers of this element. Why, for example, does the 

 linking of iodine to the third and fifth carbon atoms of the ben- 

 zene ring in the tyrosine molecule transform this inert (so far 

 as metamorphosis is concerned) amino acid into a highly active 

 agent. What is it in casein, albumen, globulin or tyrosine that 

 raises so greatly the reactive powers of iodine? 



There can be little doubt that elemental iodine when it induces 

 the transformation of thyroidless and pituitaryless tadpoles 

 combines either with the proteins of the alga? fed along with it, 

 or within the body of the tadpole after absorption through the 

 skin or alimentary tract, but it doesn't combine with anything 

 produced by thyroid tissue because there was none present nor 

 had there ever been any present in the case of the thyroidless 

 anurans. 



Hirschler ('22) precociously metamorphosed tadpoles by in- 

 serting small pieces of elemental iodine into the body of the 

 larvae. However, since the animals possessed intact thyroid 

 glands their activity can not be ruled out in this experiment, 



