240 W. W. SWINGLE. 



thyroidless and pituitaryless R. sylvatica larvae were used. One 

 set of animals was reared in tyrosine solution, another in iodo- 

 tyrosine, a third set in dibromtyrosine of twice the strength of the 

 iodotyrosine solution. No food of any kind was given. The iodo- 

 tyrosine cluture metamorphosed within fifteen days, the animals 

 of tyrosine and dibromtyrosine culture showed no indications of 

 metamorphosis after forty-three days. However, the larvae 

 reared in the dibromtyrosine solutions had well developed hind 

 legs averaging 7 mm. total length as compared with an average 

 of 3.5 for the animals of the tyrosine culture. Since no food of any 

 kind was given the increase in limb growth was probably due to 

 a slight influence exerted by the bromine ion. 



The result of this experiment upon Amblystoma punctatum 

 larvae is identical with that obtained with Spelerpes, and shows 

 clearly that it is the iodine in the tyrosine molecule that is 

 responsible for the precocious metamorphosis. The rapidity with 

 which the metamorphic response to iodotyrosine is evoked in 

 salamander larva?, would seem to indicate that the action of the 

 iodized amino-acid is directly upon the cells and tissues of the 

 organism and not necessarily through the intermediation of the 

 thyroid gland. It will be recalled, that young thyroidless axolotls 

 (Amblystoma tigrimim) transform following injections of iodo- 

 tyrosine and it is probable that thyroidless Amblystoma punctatum 

 would react likewise. However, no experiments w r ere made to 

 test this point. 



The writer now has under way a series of experiments upon the 

 metamorphosis of thyroidectomized salamander and anuran larvae 

 in which various iodized proteins and amino acids are fed and 

 injected. The results will be communicated later. 



IV. EXPERIMENTS ON Rana clamitans TADPOLES. 



The larval life of the green frog, Rana damitans, extends over 

 a year (370-400 days according to Wright, '14) and in some 

 individuals is prolonged two years before metamorphosis occurs. 

 This form offers exceptional opportunities for experimentation 

 because of the long duration of larval existence, and is especially 

 valuable for use in investigations on metamorphosis where the 

 feeding method is employed. Tadpoles of all sizes are obtainable 

 at any season of the year and when brought to the laboratory 



