TIME-RELATIONS IN AMPHIBIAN META- 

 MORPHOSIS, WITH SOME GENERAL 

 CONSIDERATIONS 



By JULIAN HUXLEY, M.A. 

 Fellow of New College, Oxford 



There have been three periods in the study of Amphibian 

 metamorphosis. In the first, workers were concerned mainly 

 with the morphology of the process, and the few experimen- 

 talists who, like Barfurth (3), attacked the problem did not arrive 

 at far-reaching or conclusive results. 



The second period we may term that of qualitative experi- 

 mental study. It was initiated by the discovery of Guder- 

 natsch (12) in 191 1 that a diet of thyroid induced precocious 

 metamorphosis in frog-tadpoles. This is now merging into a 

 third, also experimental, but aiming at quantitative results. 



It is the main purpose of this paper to draw attention to 

 the problems in this quantitative method of attack ; but 

 before doing so it will be necessary to run over, with the utmost 

 brevity, the chief facts elicited in the second period (32). 

 Thyroid substance, then, whether given fresh or dried, or in 

 the form of its active principle thyroxin, metamorphoses all 

 Amphibia, with the exception of the Perennibranchiates, of 

 which Proteus and Necturus, the only forms so far tried (16, 29), 

 are insusceptible to the largest doses. Conversely, thyroid- 

 ectomy totally prevents metamorphosis. 



Inorganic iodine causes the metamorphosis of anuran tad- 

 poles, but not of some at least of the Urodela (Amblystoma 

 of various species, including the Axolotl). It will meta- 

 morphose even thyroidless anuran larvae, as Swingle has 

 shown (28), thus proving that the thyroid function is a 

 specialisation of a function of normal tissues ; but the same 

 dose takes longer to metamorphose thyroidless than normal 

 tadpoles, and there exists a threshold-concentration below 

 which acceleration of metamorphosis may still be observed 

 in normals, but no metamorphosis is produced in thyroidless 

 specimens, 



Abelin ( i ) has shown that tyramin and related substances also 



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