NATURE OF THE RETARDING INFLUENCE OF THE 

 THYMUS UPON AMPHIBIAN METAMORPHOSIS. 



By EDUARD UHLENHUTH. 



(Front the Laboratories of The Rockefeller Institute for Medical Research.) 

 (Received for publication, October 18, 1918.) 



There is a general agreement concerning the influence of the thyroid 

 upon the metamorphosis of anuran amphibian larvae. All authors 

 who reported experiments on feeding thyroid to tadpoles, regardless 

 of the locality in which they were carried on and on whatever species 

 of anura they were performed, invariably found that thyroid feeding 

 always resulted in a precocious metamorphosis. The experiments on 

 thymus feeding, however, are characterized by just the opposite 

 feature, the results being very inconstant. Thymus feeding some- 

 times resulted in retardation or entire prevention of metamorphosis; 

 sometimes it had no effect as compared with the controls. Such dif- 

 ferent action not only was obtained in the experiments of different 

 authors, performed on different species and at different places (Guder- 

 natsch obtaining positive, Swingle negative results), but even when the 

 same investigator experimented on the same species different indi- 

 viduals were affected differently by the thymus diet (Romeis) . Nev- 

 ertheless, it is certain that in some cases th3rmus feeding actually pre- 

 vented metamorphosis and resulted in giant larvae, while no such 

 effect was obtained in the controls on the normal diet. 



It seems necessary therefore to explain why thymus feeding some- 

 times does and sometimes does not prevent metamorphosis. 



The writer performed a large number of experiments on the larvas 

 of Amby stoma maculatum,^ Amhy stoma opacum, and Amhy stoma 

 tigrinum, and it was found that while the thyroid contains a specific 

 substance enforcing metamorphosis, the preventive effect of thymus 

 feeding is due to the absence from the thymus of a substance neces- 

 sary for the formation or excretion by the thyroid of the substance 

 causing metamorphosis. 



^ For nomenclature of salamanders see Stejneger, L., and Barbour, T., A check 

 list of North American amphibians and reptiles, Cambridge, 1917. 



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