3l8 EDUARD UHLENHUTH. 



paper as early as 1919. 14 The discovery of the factor necessary 

 to release the thryoid hormone was summed up in the following 

 statement, 14 (p. 476): "Hence, besides iodine, still another sub- 

 stance is needed in the amphibian metamorphosis; namely the 

 'excretor substance' which causes the thyroid to excrete the 

 stored-up iodine." Swingle's statement 15 (p. 600), made in 1919 

 in reference to this work and quoted above, shows that he did not 

 recognize the existence of such a releasing factor. 



Since organs of internal secretion or any other organs would 

 manifest themselves physiologically in a manner essentially 

 similar to a substance and since it seemed undesirable to reflect 

 in the term applied to the releasing factor upon any preconceived 

 theory, the term "excretor substance" was replaced later on by 

 the term "releasing factor" 16 (p. 207) and "releasing mecha- 

 nism" 10 (p. 112), both of them implying merely the function by 

 which this factor manifests itself and which was actually observed. 



Since the first communication was made my experiments were 

 continued and it was shown, for 3 different species of salamanders, 

 that in low temperature metamorphosis is greatly retarded in 

 proportion to general growth, while the development of the 

 thyroid gland shows no such retardation. It was concluded from 

 this fact that since the thyroid developed at a normal rate in 

 proportion to general growth the development of the releasing 

 mechanism was retarded. Several papers were published in 

 regard to this problem and the difference between the retardation 

 of the thyroid and the releasing mechanism in response to the 

 same degree of lowered temperature was explained by assuming a 

 lower temperature coefficient for the thyroid than for the releas- 

 ing mechanism. In 1921 the results of this work were summarized 

 in the following statement 16 (p. 206): "The most conspicuous 

 character in the salamander metamorphosis is the fact that 

 although it certainly is dependent on the thyroid hormone, it does 

 not necessarily take place in larvae whose thyroid is mature. This 

 can only mean that two factors are required in order to bring 

 about the metamorphosis of salamander larvse, namely a mature 

 gland and a factor which releases the thyroid hormone from the 

 follicles of the gland." 



Further confirmation of the existence of a releasing mechanism 



