.W. W. SWINGLE 167 



By July 30, all the normal iodine-fed animals in Cultures 1,2, and 

 3 had completed metamorphosis, while in Culture 4 many of the 

 larv£e were transforming, and all had completed the process by August 

 2. Of the thyroidless animals. Cultures 1 and 2 had metamorphosed 

 earlier, but Cultures 3 and 4 showed on this date no very marked 

 indications of assuming adult characters very soon. These two cul- 

 tures were fed and tended as usual until September 5. During this 

 interval the animals increased considerably in size, those in Culture 3 

 averaging 52 mm. with hind legs 10 mm. long; the larvse of Culture 4 

 averaged 47 mm. with hind legs 7.5 mm. There were no marked 

 indications of metamorphosis. The two control cultures, one normal 

 and the other thyroidless, averaged in length 27.5 mm. and 40 mm. 

 respectively, with hind legs 7.5 mm. and 5.5 mm. Growth in these 

 two cultures was very slow after July and practically ceased by 

 August 2. The diet appears responsible for this, in part at least. 

 In earlier papers^ the writer has shown that when fed in minimum 

 quantities iodine stimulates growth in anuran larvae. This was 

 also found to be the case in the present experiment. The growth 

 rate of frog larvae reared artificially is so variable, however, the 

 writer lays no great stress upon this point. 



The history of thyroidless Cultures 3 and 4 leads to the conclusion 

 that 0.0000007666 and 0.0000003833 concentrations of elemental 

 iodine are insufficient to induce complete metamorphosis in animals 

 devoid of the thyroid apparatus, even when fed over fairly long 

 periods of time. The organism is apparently unable to utilize the 

 minute quantities of iodine absorbed through the integument without 

 the aid of the thyroid mechanism to collect and store it, thus pre- 

 venting its passage through the body unused. This is good evidence 

 that larvae with intact thyroid apparatus are capable of utilizing very 

 much smaller quantities of elemental iodine than are thyroidless 

 animals. The lower limit of the iodine concentration requisite for 

 metamorphosis of normal larvae was not determined, as 0.0000007666 

 and 0.0000003833 were the weakest used and both proved to be 

 above the lower limit just necessary to induce metamorphosis. On 

 the other hand, these same concentrations were too low to induce 

 metamorphosis in thyroidless animals. 



The determination of the exact amount of iodine absorbed by the 



