W. Vv^. SWINGLE 



165 



Observations of June 25. 



* Fore limbs through skin in six animals; culture completed metamorphosis by 

 une 29. Four animals died before resorption of the tail was complete. 



It will be noted that Cultures 1 and 2 of the normal iodine-fed lar- 

 vae, living in iodine concentrations of 0.0000014532 and 0.0000011499 

 respectively, completed metamorphosis before the corresponding 

 thyroidless cultures began to transform. It will be shown later 

 that the interval between metamorphosis of normal iodine-fed and 

 thyroidless iodine-fed cultures in equivalent solutions is lengthened 

 the lower the iodine concentration, finally becoming perhaps an 

 indefinite interval in the case of the thyroidless animals.^ This 

 strongly indicates that there is an irreducible minimum of iodine con- 

 centration for metamorphosis, beyond which thyroidless larvae are 

 'Unable to utilize the minute quantities of iodine present, though this 

 may be sufficient to stimulate metamorphosis in animals with intact 

 thyroid apparatus. 



^Thyroidless Cultures 3 and 4 unfortunately were lost September 11, owing to 

 neglect to change the water for 52 hours, resulting in decay of the food, especially 

 casein, and consequent death of the larvae. At this time none of the animals in 

 Culture 4 showed indications of metamorphosis. However, two animals of 

 Culture 3 were about to transform. 



