482 THYROID, METAMORPHOSIS, AND GROWTH 



The same phenomenon was observed also in the thymus-fed larvae 

 (Series D 1916, Table II) and since here the lack of iodine and that 

 of the excretor substance are combined, it is not surprising that 

 the value for R X A is still higher (28 per cent) above the aver- 

 age than in the low temperature series of the worm-fed larvae (Series 

 C 1916, Table I). 



Thus with the hypothesis of an excretor substance we can explain 

 a phenomenon which for a long time was confusing to experimental 

 biologists as well as to systematists. 



SUMMARY. 



1. Two substances are involved in amphibian metamorphosis as 

 studied in Ambystoma opacum: first, iodine, which is taken up by the 

 food, and second, an excretor substance, which is evolved during the 

 processes of growth and serves to induce the excretory function of the 

 thyroid gland. 



2. This explains why in larvae, whose metamorphosis is inhibited 

 by lack of iodine, growth is checked at the time when metamorphosis 

 should occur; for at this time the excretor substance commences to 

 act and this results, if iodine is absent, in the excretion by the thy- 

 roid of toxic substances which cause the breakdown of proteins and 

 consequently a decrease in size of the larvae. 



3. Larvae whose metamorphosis is inhibited by extirpation of the 

 thyroid or by the hereditary lack of a thyroid (as is the case in Ty- 

 phlomolge) can grow normally, since in them the action of the excretor 

 substance cannot result in the excretion by the thyroid of a toxic 

 growth-inhibiting substance. 



4. At low temperature less excretor substance is produced than at 

 high temperature during an equal rate of growth; therefore larvae kept 

 at low temperature reach a larger size than larvae kept at high tem- 

 perature, before they metamorphose. 



The writer wishes to express his thanks to the Library of the Brook- 

 lyn Museum, and especially to Miss S. H. Hutchinson, for courtesies 

 extended to him. 



