424 W. W. SWINGLE 



termined by the hereditary constitution of the organism. Bufo 

 metamorphoses after a few weeks, Rana pipiens requires at 

 least two months; Rana catesbiana is said to require two, three, 

 and sometimes four seasons for complete metamorphosis. The 

 difference in the time required by these amphibians to meta- 

 morphose is very great, and yet at certain stages in their life 

 history they may all be found living together in the same pool. 

 It is obvious that the hereditary factors controlling the growth 

 processes play a very great role in determining the time of 

 metamorphosis, and that iodin is simply an initiator of the 

 process. 



SUMMARY AND CONCLUSION 



1. The thyroid glands of iodin-fed frog larvae are larger than 

 the glands of control animals held at the same body length as 

 the animals of the iodin-fed culture by underfeeding. 



2. The follicles of the glands of such iodin-fed larvae contain 

 much greater colloid mass than the follicles of the controls. 



3. Solutions of iodin will bring about metamorphosis in both 

 normal and thyroidless tadpoles in a short time. 



4. Iodin is much more active in accelerating metamorphosis 

 than any of its compounds. Next in order of activity are iodo- 

 fonn, potassium iodide. Potassium iodate appears to have 

 no effect. 



5. The suggestion is made that amphibian metamorphosis is 

 a result of the interaction of environmental agencies, such as 

 iodin and its compounds, with the hereditary factors controlling 

 the growth processes. 



