234 w - w - SWINGLE. 



the iodotyrosine culture showed marked gill and tail-fin reduc- 

 tion. None of the other animals of this or other cultures showed 

 any change. However, by November 5, i.e., twenty days from 

 the first administration of iodotyrosine, all of the larva? kept in 

 solutions of this substance had metamorphosed. The external 

 gills had disappeared, the gill clefts had closed and the tail fin was 

 completely resorbed. The animals left the water and crawled up 

 the sides of the container. 



Examination of the tyrosine and dibromtyrosine cultures 

 showed no indications of metamorphosis. The animals of these 

 cultures were kept in the solutions for a month longer but meta- 

 morphosis was not induced. At the close of the experiment 

 exceedingly strong concentrations of tyrosine and dibromtyrosine 

 were employed but with negative results. The larvae of the 

 normal culture kept in the large aquarium and plentifully supplied 

 w r ith food likewise failed to metamorphose or to show any indica- 

 tions of transformation a month after the animals of the iodo- 

 tyrosine culture had completed the process. 



The results of this experiment are quite clean cut and admit of 

 but one interpretation : It is the iodine within the tyrosine mole- 

 cule that is responsible for the induced metamorphosis because 

 the tyrosine and dibromtyrosine were ineffective either in weak 

 or strong concentrations when administered over comparatively 

 long periods. 



Efforts were made to thyroidectomize the larva? but without 

 success owing to the extremely small size of the thyroid glands. 

 The thyroid apparatus of Spelerpes larvae of 47 mm. total length 

 does not contain enough of the physiologically active hormone 

 to induce metamorphosis in anuran tadpoles when heteroplasti- 

 cally transplanted. This experiment was attempted by Mr. O. 

 M. Helff of this laboratory but without success. 



Judging by the positive results obtained w T ith thyroidless 

 axolotls, it is highly probable that thyroidless Spelerpes would 

 react by rapid metamorphosis if injected with, or reared in solu- 

 tions of iodotyrosine. It is interesting to note that both large 

 and small larvae metamorphosed within twenty days, though the 

 differences in size were great, varying as they did from 23 mm. 

 to 52 mm. total length. Probably some of the smaller animals 

 were considerably younger than the larger ones, however, it is 



