RELATION OF lODTN TO THYROID 411 



thyroidism appear a few days after administration of the sub- 

 stance. Overfeeding with iodin has the same result as over- 

 feeding with thyroid extract — death of the organisms from a too 

 rapid rate of metabolism. Iodin, if carefully administered, will 

 stimulate metamorphosis in a shorter time than the fresh gland 

 tissue. Two years ago the writer fed fresh thyroid gland to 

 frog larvae and kept a record of the rate of body change and 

 the time required for such changes to occur when the larvae were 

 fed fresh tissue and the powdered extract. A comparison of 

 these time intervals with those of the iodin-fed animals of the 

 present experiment show clearly that the fresh gland tissue is 

 not so effective in inducing metamorphic change as the inorganic 

 iodin. The animals used in both cases were of the same species. 



EXPERIMENTS WITH FEEDING THYROIDECTOMIZED LARVAE 



IODIN 



Since normal larvae were found to react to iodin feeding by 

 marked metamorphic changes, it was considered worth while to 

 carry out the same experiment upon animals whose thyroids had 

 been removed during early embryonic life. The work of Allen 

 ('18) has shown that thyroidectomized larvae fail to undergo 

 metamorphosis, but instead per'manently retain their larval 

 characters. If such thyroidless animals could be induced to 

 metamorphose under the stimulus of iodin feeding, new light 

 would be shed upon the iodin-thyroid problem and upon the 

 causes of amphibian metamorphosis. 



Eighty very young toad tadpoles, the thyroid glands of which 

 had been removed at the 4-mm. stage by Prof. B. M. Allen, of 

 the University of Kansas, were obtained through the generosity 

 of this investigator when the larvae averaged 6.5 mm. in length. 

 The animals were fed agae until they were 10 mm. long and then ' 

 fed iodin crystals and flour. This series was controlled by both 

 normal and other thyroidectomized larvae of the same age. The 

 iodin mixture was first administered the 27th of May. Ten 

 days later, examination of the culture revealed well-developed 

 limbs on all of the thyroidless larvae. The limbs were visible 

 without the aid of a lens and had differentiated toe points. The 



