No. 638] INTEBNAL tiKCTlETIONS OF AMPHIBIANS 201 



stone caves ; nothing, however, is known about the endo- 

 crine glands of this animal. 



If- the thyroid substance is capable of causing tlio de- 

 velopment of the characters of a terrestrinl aniiiliil^ian, 

 the administration of thyroid substance >li(>ul(] cause 

 metamorphosis of Proteus anguineus. .Jensen (14) 

 subjected Proteus to the action of thyroid substance, but 

 did not get any demonstrable results. Many causes may 

 have been responsible for this failure, in particular the 

 fact that the animals were too old when they wore sub- 

 jected to the thyroid feeding. 



It has been known for some tiiue that tlic ctTi ct of eunal 



doses of thyroid substance on the amphibian nietaiiKU'- 

 phosis is the greater, the more iodine there is contained 

 in the thyroid gland (15). Recently, Swingle (16) has 

 demonstrated that the feeding of common inorganic 

 iodine to tadpoles or the keeping of the tad])oles in iodine 

 solutions accelerates metamorphosis in the same way as 

 does the thyroid. This effect of iodine is strictly (luaiiti- 

 tative; if there is no iodine contained in the food of the 

 tadpoles, metamorphosis is inhibited, while with an in- 

 creasing amount of iodine metamorphosis is increasingly 

 accelerated. Moreover, the effect on the relation between 

 growth and development is the same in iodine solutions 

 and in thyroid feeding. Weak solutions of iodine increase 

 not only the rate of development, but also tlie rate of 

 growth, while high concentrations prevent growth. 

 There can be no doubt that at least in the metamorphosis 

 of tadpoles, iodine is an indispensable constituent of the 

 thyroid hormone. 



Swingle (16) found that potassium iodide and iodo- 

 -form had an effect on nu'tainorphosis similar to tliat of 

 iodine, while bromine had no effect on metanioi-phosis 

 and growth. Thus the effect of lodin.^ appears to l)e very 

 specific when comparison is made with so nearly related 

 a substance as bromine. 



The feeding of iodine to nianunalians does not produce 

 the same effects as the administration of thyroid sub- 



