422 W. W. SWINGLE 



line has shown that both normal or thyroidless frog or toad 

 larvae will undergo metamorphosis very quickly if placed in 

 extremely weak solutions of iodin. The defect in the earlier 

 work was that the solutions were too strong. Just a trace of 

 iodin in the water is sufficient to produce results if the solution 

 is kept fresh. Cultures of tadpoles fed on wheat-flower paste 

 showed no changes whatever when compared with beef-fed or 

 algae-fed controls. This experiment shows clearly that iodin is 

 the active principle of the mixture of flour, iodin, and water, fed 

 in the previous work, and that the flour has nothing to do with 

 the results obtained. 



Thg fact that thyroidless tadpoles readily undergo meta- 

 morphosis when fed iodin led the writer to suggest that the 

 function of this gland is chiefly that of iodin storage, rather than 

 the elaboration of a specific hormone, and, moreover, that the 

 tissues of animals are capable of utilizing iodin directly without 

 the intermediation of the gland. In this connection the results 

 of tests made to determine the solubility of iodin in normal 

 blood serum may be of interest. The serum of amphibians and 

 mammals was used; the amphibian serum was obtained from 

 adult Rana pipiens, the mammal serum from rabbits. The 

 serum of the latter at 37°C. acts as a solvent for finely ground 

 iodin crystals to the extent of 0.00075 gram per cubic centi- 

 meter when stirred vigorously. The solvent powder of Rana 

 pipiens serum is somewhat less than that of rabbits, though 

 considerably more than that of water. 



DISCUSSION 



The iodin-feeding experiments described in this and the pre- 

 ceding paper should prove of interest to students of amphibian 

 metamorphosis, as they give a clue as to the nature of one of 

 the underlying causes of this phenomenon. It has been assumed, 

 and probably correctly so, that one of the prime requisites' of 

 the change from the larval to adult condition in Anurans is a 

 heightened metabolism. The work of Gudernatsch with feeding 

 thyroid to tadpoles showed that this substance accelerated meta- 

 morphosis, and it is generally agreed that the efl'ect of thyroid 



