Nature of the Retarding Influence of the Thymus upon Amphibian Metamorphosis. by Eduard Uhlenhuth

There is a general agreement concerning the influence of the thyroid upon the metamorphosis of anuran amphibian larvae. All a~thors 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 sometimes resulted in retardation or entire prevention of metamorphosis; sometimes it had no effect as compared with the controls. Such different action not only was obtained in the experiments of different authors, performed on different species and at different places (Gudernatsch obtaining positive, Swingle negative results), but even when the same investigator experimented on the same species different individuals were affected differently by the thymus diet (Romeis). Nevertheless, it is certain that in some cases thymus feeding actually pre~ vented metamorphosis and resulted in giant larvae, while no such effect was obtained in the controls on the normal diet. I t seems necessary therefore to explain why thymus feeding sometimes does and sometimes does not prevent metamorphosis. The writer performed a large number of experiments on the larvm of Ambystoma maculatum, 1 Ambystoma opacum, and Ambystoma 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 necessary for the formation or excretion by the thyroid of the substance causing metamorphosis.