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THE AMERICAN NATUBALIST [Vol. LV 



stance. This fact has formed the ])asis for the opmion 

 (7) that the characteristic action of the thyroid hormone 

 is not directly caused by the presence of iodine in the 

 thyroid hormone. It seems, however, more probable that 

 the feeding of iodine has no effect on mammalians, be- 

 cause the mammalian organism, for some reasons, can 

 not utilize an excess of iodine. It is well kno\\m that the 

 mammalian thyroid gland is capable of storing large 

 amounts of iodine (17). If, under normal conditions, 

 only a definite amount of hormone could be excreted by 

 the thyroid gland, the feeding of excess amounts of iodine 

 would have no effect in the healthy individual, since every 

 excess of iodine would be retained and stored by the thy- 

 roid tissue. If in the mammalian organism the thyroid 

 gland should be the only organ capable of elaborating the 

 thyroid hormone, the feeding of iodine could have no 

 effect in the absence of the thyroid, or in persons whose 

 thyroid function is insufficient. Conditions are different 

 with tadpoles. Swingle (16) has shown that even in 

 thyroidectomized tadpoles, iodine solutions are capable 

 of causing metamorphosis. Apparently the thyroid 

 gland is not the only organ of the tadpole which can pro- 

 duce the thyroid hormone.- 



It should be pointed out, however, that a fundamental 

 difference exists between frogs and toads on the one 

 hand, and salamanders on the other, as regards their 

 reaction to iodine. Salamanders behave much like mam- 

 malians. Although I was able to confirm the accelerating 

 action of iodine at least in the development of the limbs 

 of the tadijolos, I htwo not been able to cause p.-eeocious 

 metamorphosis by placiiii^- salainiuidcr lar\;e in iodine 

 solutions. Tahlr II will i llur^t nitr this >tatrni.Tit. 



Two larva' nf tin- ^in.-i,.. .1. nun ulannn w.iv k.-pt tirst 

 in a solution of dr..p< ' ni. io.liiir pt-r I.OIH) c.e. water 

 and then, up to metaiiiMi plio^is, in a :;-drops-iodine solu- 

 tion. No acceleration of mctanioi pliosis took place; the 

 larvfP metamorphosed at an age of ]'2'2 days, while the 

 controls were only lol days old when thry iiictamor- 



