6 



The Journal of Heredity 



The final startling discovery Ijy Swin- 

 gle, that inorganic iodine will meta- 

 morphose even thyroidless larvae, at 

 first sight seems to militate against this 

 view. However, as Swingle himself 

 pointed out, it only implies that the 

 general tissues of the body possess 

 to some extent qualities which have 

 been specialized in the thyroid tissue. 

 Uhlenhuth^'^' i^- justly points out that 

 w^e know nothing of the fate of the 

 iodine in the body and that it is easy to 

 suppose that it is worked up as to 

 believe that it acts directly. As a mat- 

 ter of fact, it is easier to believe the 

 former, since Swingle's own results 

 showed that thyroidless animals meta- 

 morphosed less rapidly than normal 

 ones in the same concentration of 

 iodine. This would be the case if the 

 same processes were occurring in both, 

 only less rapidly in the thyroidless ones, 

 whereas it would not be so if the total 

 concentration of iodine were the de- 

 termining factor. 



Metabolic Indicators 



It must always be remembered that 

 in the adult mammal complex self- 

 regulating mechanisms are at work, 

 which tend to keep the production of 

 the active secretion of any gland with- 

 in comparatively narrow limits. In 

 the developing Amphibian, however, 

 such regulatory machinery appears 

 not to exist, so that it is easy to ob- 

 tain a relative increase in the size and 

 productiveness of a particular ductless 

 gland. In the tadpole, therefore, iodine 

 above the normal is incorporated into 

 the thyroid ; whereas, in the healthy 

 adult mammal, the balance, after a 

 comjiaratively small amount has been 

 taken up by the thyroid, is excreted. 



Taking all these facts into considera- 

 tion, it is fair to say that in Anura, 

 if)dine, during larval life at least, is a 

 limiting factor for the growth of the 

 thyroid, so that with more iodine, more 

 of the iodized organic compound is 

 formed ; further, that in thyroidecto- 

 mized animals, the same or a similar 

 organic compound is formed, but that 



it cannot Ije formed so rapidly or in 

 such large quantities by the unspecial- 

 ized general tissues as by the thyroid, 

 which has been specialized for this 

 particular function. 



Further, the failure of iodine alone 

 to produce metamorphosis in Axolotls 

 and at least some other Urodela, where- 

 as thyroid is efl:'ective, is impossible 

 to reconcile with the hypothesis. 



The evidence at present strongly in- 

 dicates that inorganic iodine exerts any 

 metamorphic efiiects it may have in 

 Amphibia in the same way as it exerts 

 any therapeutic effects it may have in 

 Alammalia, indirectly, through the thy- 

 roid gland ; and the onus of proving 

 that the substance which exerts an 

 effect upon Amphibian metamorphosis 

 is not the same as that which causes 

 a rise of basal metabolism in mam- 

 mals, rests on those who propound 

 such a theory. 



This is of considerable importance, 

 since, if the effect of thyroid upon 

 Amphibian! metamorphosis is of the 

 same natm^e as its effect upon human 

 basal metabolism, the Amphibia, so 

 soon as an accurate quantitative method 

 is devised, can be used as "metabolic 

 indicators," and will be of the greatest 

 value as such ; whereas they will not 

 have this value to physiological science 

 if the thyroid secretion acts on them 

 in a different way from what it does in 

 mammals. 



If it is possible to have tissues which 

 can only maintain themselves below a 

 certain level of metabolism, it is equal- 

 ly possible to have similar tissues 

 adapted to a higher rate, and thus not 

 breaking down imtil a higher relative 

 concentration of thyroid secretion is 

 reached in the organism. It should 

 equally well he possible to have tis- 

 sues which did not undergo general 

 breakdown ( apart from some wasting 

 away due to increased metabolism) 

 under the influence of the greatest 

 concentration of thyroid substance 

 artificially producible in the organism, 

 any more than do the adult tissues of 

 Amphibia and those of mammals as 



