RELATION OF lODIN TO THYROID. 421 



are just half the size of the animals of the well-fed culture of 

 controls, it is clear the iddin-fed animals have relatively 

 much the larger glands. Microscopic examination of the colloid 

 content of the glands of the experimental and the two control 

 cultures of larvae, shows a marked difference in the amount of 

 colloid visible in the folhcles. The glands of the iodin-fed 

 animals were packed with this substance, whereas the glands of 

 the controls showed a rather scanty amount. 



Since the completion of part I of these studies, the writer has 

 carried out several more iodin-feeding experiments in order to 

 test various points left untouched in the earlier work. One of 

 these points barely touched upon was the comparative rapidity of 

 action of the various iodin compounds in accelerating meta- 

 morphosis in normal and thyroidless tadpoles. A detailed ac- 

 count of the experiment will not be given here, as it was for the 

 most part a repetition of the experiments described in the earlier 

 paper. Suffice to state here that iodin crystals when fed to frog 

 or toad larvae with and without thyroid glands bring about 

 metamorphic changes in the larvae much more rapidly than any 

 of the compounds used; iodoform is somewhat slower in its 

 action, but is much more rapid than potassium iodide. Three 

 feeding experiments were carried out in which potassium iodate 

 was used as food, but as only negative results were obtained the 

 conclusion is justified that this compound has no accelerating 

 effect upon metamorphosis. The larvae eat the substance, but 

 apparently are unable to break it down sufficiently to release 

 free iodin. 



W^iile engaged in the, experimental work which formed the 

 subject-matter of the previous paper, the writer was under the 

 impression that perhaps the results obtained from feeding iodin 

 to tadpoles were due to the mixture of flour, iodin, and water 

 used, and not entirely to the iodin itself. This erroneous idea 

 was due to the fact that in several earlier experiments made 

 to test this point it was observed that frog larvae die very quickly 

 if placed in containers with inorganic iodin crystals or in weak 

 solutions of this substance. However, further work along this 



