210 COMPARATIVE PHYSIOLOGY 



iodine, and that administration of tlie gland substance com- 

 pensates for the clinical disturbances resulting from its removal 

 or disorder — notably a reduction in basal metabolism. 

 Recently Kendall has isolated a substance having the properties 

 of thyroid extracts and the constitution of a tri-iodo derivative 

 of tryptophane. Helff (1923) and Huxley (1925) have shown 

 that thyroid extract increases respiratory exchange in the 

 tadpole ; but this does not appear to be the case in the adult ; 

 and Champy (19 19) claims that the thyroid hormone acts 

 selectively on larval tissues. The significance of the thyroid 

 has been further explored by Swingle (19 19) and by Uhlenhuth 

 (1921 and 1922). Swingle's observations concern the relation 

 of the organism to its iodine supply : the iodine content of 

 water and food is the limiting factor in thyroid develop- 

 ment and consequently in metamorphosis. This observation 

 throws a flood of light upon a phenomenon of no little bionomic 

 interest — neoteny in Anura. Neoteny in urodeles, however, 

 is a different matter. Two different grades are illustrated in 

 Amblystoma tigrinum by the European strain of the Mexican 

 species and the species from Colorado. The former never 

 undergoes metamorphosis in aquaria ; the latter is easily 

 induced to transform into the terrestrial salamander form by 

 external disturbances of one kind or another. Inorganic 

 iodine administered to urodele larvae does not induce metamor- 

 phosis as was shown by Swingle to be the case in frog tadpoles. 

 The experiments of Uhlenhuth and of Swingle (1922) show that 

 the axolotl thyroid, while containing the thyroid hormone, 

 requires some special stimulus to bring about its discharge 

 into the blood stream. Neoteny may be due then to lack of 

 sufficient iodine in the environment or inadequacy of the 

 mechanism which controls the discharge of the thyroid 

 hormone. It may finally be asked whether either of these 

 explanations extend to the true perennibranchiate genera 

 (Proteus, Necturus, Typhlomolge, and Siren). The first 

 three (Jensen, Huxley and Hogben, Uhlenhuth, Swingle) have 

 not been found to respond to the action of the thyroid hormone 

 at all. But since it is almost certain (i) that thyroid does not 

 increase basal metabolism in adult frogs ; (2) that the action 



