204 THE AMERICAN NATURALIST [Vol. LV 



summer, were subjected to an iodine treatment. They 

 were placed in water containing 5 drops of a V20 m. solu- 

 tion of iodine per 1,000 c.c. of water and, as they showed 

 no reaction of any kind, this concentration was increased 

 gradually to 8 drops and in one larva to even 13 drops of 

 iodine (3 drops of a V20 m. solution of iodine per 1,000 c.c. 

 of water is enough to cause growth of the hind limbs in 

 larvai of Rana sylvatica), which is more than 0.2 c.c. of 

 a %o m. solution of iodine per 1,000 c.c. of water. Although 

 these lar\'ae have now been in the iodine solution for 2 

 months, none of them has developed any tendency 

 towards metamorphosis, while 3 other control larvae, 

 among them a neotenous specimen, metamorphosed 13 

 days after being placed in an emulsion of 0.1 gm. of 

 Bayer's iodothyrine per 1,000 c.c. of water. Evidently 

 the assumption suggested by Swingle (16, III), that lack 

 of iodine prevailing in the lakes is causing the inhibition 

 of metamorphosis of the axolotl and other urodelans, is 

 unwarranted. I will show presently that in the inhibition 

 of metamorphosis and in neoteny of axolotls and probably 

 certain P^uropean urodelans we are confronted with an 

 entirely new phase of internal secretion, namely with the 

 differential action of temperature upon the development 

 of various components of the endocrine system. 



In a former article (20) I suggested the Inhi- 



bition of metamorphosis in thymus-fed auiplilltian \:wmv 

 may be caused by lack of iodine in the tliynius. Swi ugU^ 

 (16, III) has accepted and unfortunately repeated, with- 

 out further testing, this suggestion. But recent experi- 

 ments show that tliis view must be abandoned, since 

 addition of iodine to a ])nre thymus diet does not enable 

 the s;il;nu;ni(lci- citlifi- to grow or to metamorphose. 



Siniilarix- the i-t t;i idat ion of growth and metamorphosis 



the water. 



The iodine re-iuimnent of sah-inian(h'rs ninst be ex- 

 tremely slight, since anterior lobe of hypophysis, a nearly 



