RELATION OF lODIN TO THYROID 423 



extract on tadpoles is accomplished chiefly by greatly accelerating 

 catabohc activities. The writer in 1915-1916 (results published 

 in 1918) in an experiment to test the effects of inanition upon 

 the development of the germ cells and germ glands of frog 

 larvae, found that starvation totally inhibits all body growth 

 and differentiation, the animals consequently never assuming 

 the adult condition. Such prolonged starvation undoubtedly 

 acts as a depressor of metabohsm. Later Allen observed that 

 thyroidless tadpoles do not undergo metamorphosis, but instead 

 grow abnormally large. In this case the absence of the thyroid 

 function had led to a prolonging of the anabolic phase of the 

 metabolic activities. In part I of this series of iodin studies it 

 was shown that iodin accelerates metamorphosis in both normal 

 and thyroidless tadpoles. The iodin effect like that of the 

 thyroid tissue or extract (and indeed iodin seems to be the 

 active principle of the thyroid) is in heightening catabolism. 



In these four experiments there is fairly good evidence for the 

 view that amphibian metamorphosis was due, in part at least, to 

 heightened metabolism of the catabolic type. In a state of 

 nature the metamorphosis of frog larvae, is under normal con- 

 ditions, effected by none of the experimental agencies mentioned, 

 except iodin. This substance is found in many plants (though 

 perhaps accidentally present, as some authors believe) ; it is 

 present in the soil in combination with other substances and 

 present in the thyroids of most animals. Iodin in some form or 

 other may be said to constitute a normal environmental factor 

 of amphibians, a factor which, when considered in connection 

 with the hereditary factors governing growth processes in larval 

 amphibians, gives a rationale of the factors involved in the 

 metamorphosis of these animals. 



As pointed out by Morse ('18), it is impossible to bring about 

 complete metamorphosis in extremely young larvae, when an 

 attempt to do so is made by feeding large quantities of thyroid 

 or iodin, the animals die. Marked metamorphic changes appear, 

 but death usually supervenes before complete transformation 

 takes place. A certain cycle of events must take place before 

 metamorphosis, and this normal cycle is in all probability de- 



