480 THYEOID. METAMORPHOSIS, AND GROWTH 



If prevention of metamorphosis by the absence of iodine, but in the 

 presence of a thyroid gland, is accompanied by a check in growth 

 (for the reasons given above), we should expect that prevention of 

 metamorphosis in the absence of a thyroid gland would not be accom- 

 panied by an abnormal check of growth. Hence amphibian larvse 

 deprived of their thyroids should be able to complete the normal 

 growth of the species without any disturbances. That this is actu- 

 ally the case is shown in Allen's tadpoles which were deprived arti- 

 ficially of their thyroid glands and which reached frequently enormous 

 sizes, becoming real giant larva?. A similar phenomenon is found in 

 such forms as Typhlomolge rathhuni, which for some reason do not 

 develop a thyroid gland — as discovered by Emerson^ — and hence 

 remain permanently in a larval condition, but which seem able to 

 grow for years without disturbances. 



Thus the experiments reported in this article seem to prove that 

 metamorphosis of the normal larvae of Amhy stoma opacum which 

 possess a thyroid depends not only upon the presence of a sufficient 

 amount of iodine in this gland but also on the presence of the action 

 of a second substance inducing the excretion of the iodine by the 

 thyroid gland. By means of this hypothesis we are able to explain 

 why thymus-fed larvae suddenly stop growing at the time when meta- 

 morphosis should occur, without, however, metamorphosing, and 

 why species not possessing thyroid glands, such as the Typhlomolge, 

 can complete their growth without disturbance. 



Metamorphosis and Temperature. 



There is still another phenomenon which lends itself readily to 

 explanation on the basis of the assumption of an excretor substance. 

 It is a well known fact that growth is retarded at low temperature, 

 and since we have seen that the excretor substance is evolved during 

 growth, it is not surprising that metamorphosis also should be re- 

 tarded at low temperature. This has long been observed by many 

 students of amphibian metamorphosis. But what remains unex- 

 plained is the fact that amphibian larvae when kept in low tem- 

 perature are always much larger at the time of metamorphosis than 



'' Emerson, E. T., Proc. Boston Soc. Nat. Hist., 1905, xxxii, 43. 



