1. The standardisation of the method of preparation of extracts and the adoption of a uniform procedure for the estimation of their ability to induce metamorphosis in axolotls or accelerate the transformation of tadpoles to adults permit a biological assay of the metamorphic activity of extracts of the anterior lobe.

2. The metamorphic activity of the anterior lobe is specific, since similar tissue extracts fail to excite a like response and qualitative and quantitative characteristics distinguish it from the activity of the thyroid in metamorphosis.

3. Active preparations are not produced when neutral media or inorganic acids are used for extraction from fresh gland by decoction.

4. The potency of extracts depends upon the strength of the acetic acid used for extraction, the concentration of the extract and the hydrogen-ion concentration.

5. The threshold value of the dose inducing metamorphosis in axolotis by injection and the amount inducing the maximum rate of change increase with the size of the animal. Injections must be continued at regular and suitably short intervals till the completion of the change to maintain the maximum response to any particular dose.

6. The activity of extracts is destroyed by pepsin and trypsin and diminished by dilute hydrochloric acid. Neutralised extracts are inactive.

7. Ringer's and distilled water extracts of thyroid are more active than similar extracts in dilute acetic acid. There is no relation between the activity of these extracts and the hydrogen-ion concentration.

8. There is evidence of an inhibiting influence of the posterior lobe upon metamorphosis.

9. The distribution of the factor responsible for the metamorphic activity of the anterior lobe is not uniform throughout the lobe, less being present at the border than within. No definite localisation is apparent.

10. The anterior lobe influences metamorphosis and growth in size, but the same influence does not appear to operate in the production of each of these effects. The extent to which the growth stimulus is dependent upon and determined by other factors has yet to be determined.

11. The thyroid and pituitary are interdependent, but both seemingly are able to induce metamorphosis in the axolotl independently, and in the absence of the other. In the tadpole independent action in stimulating metamorphosis appears to be possible only in the later stages, co-operation being necessary in development and the earlier stages.

12. The thyroid and pituitary of the adult axolotl appear to be at a low level of functional activity, and metamorphosis is dependent upon the stimulus of glands from external sources when suitably administered, since the rate of change induced is approximately the same in these animals, whether normal, thyroidless, pituitaryless or with both thyroid and pituitary removed.

13. The relationship between the endocrine balance and metamorphosis in the amphibia and its evolutionary significance is discussed.

14. The anterior lobe appears to have a regulative and directive effect within the organism which, as a factor in the internal environment, may have some evolutionary significance.

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