I5O E. UHLENHUTII. 



In the mind of the readers not quite familiar with the amphib- 

 ian technique doubt may arise as to the correctness of the inter- 

 pretation of the metamorphosis of one of the iod'ine-fed larvae as 

 not be'ing due to the action of the iodine. I may mention, therefore 

 that all of the axolotl larvae, when kept under the conditions em- 

 ployed in the present control experiments, do undergo metamor- 

 phosis spontaneously. Consequently metamorphosis of the iod'ine- 

 fed axlotl larvae could be interpreted as the result of the iodine 

 administration only, if it would have occurred at a greatly pre- 

 cocious date and simultaneously or nearly simultaneously in all 

 the experimental larvae. Not only were these conditions not ful- 

 filled, but one of the experimental larvae did not metamorphose 

 at all and, at the termination of the experiment was farther away 

 from the metamorphosis than any of the control larvae, although 

 observation extended over a period of more than 4 months, dur- 

 ing which time large amounts of iodine were administered. 



That the dosis of iodine was too small to enforce metamor- 

 phosis, can not be assumed; for, as will be discussed presently, 

 the dosis of iodine administered to the larvae of the iodine series 

 was far in excess over the quantity of 'iodine contained in the 

 dosis of iodothyrin which caused a very rapid metamorphosis. 



Furthermore it is impossible to explain the ineffectiveness of 

 the inorganic iodine by assuming that the larvae employed in the 

 iodine experiment had entered a stage at which the respons'ive- 

 ness to the stimuli causing metamorphosis has been lost. Not 

 only could metamorphosis easily be produced by iodothyrin, but 

 occurred spontaneously in the controls and in many other axolotl 

 larvae comparable to the iodine-fed larvae in every respect (ex- 

 cept for the iodine administration). 



Hence it is evident that inorganic iodine is incapable of causing 

 metamorphosis of the salamander larvae. 



Regarding the role of the iod'ine in the thyroid hormone these 

 experiments also show, in a very striking manner, that the effect 

 of the iodothyrin is not due to its iodine content. If it were due 

 to its iodine content, quantities of inorganic iodine equal to those 

 contained in an effective diesis of iodothyrin should cause meta- 

 morphosis. This expectation, however, is not fulfilled in the re- 



