1. Though thymus-fed salamander larvæ often metamorphose normally, thymus feeding sometimes retards and in rare cases inhibits metamorphosis completely.

2. The addition of normal food to a thymus diet abolishes the inhibitory effect of the thymus.

3. Addition of a small quantity of iodothyrin leads rapidly to precocious metamorphosis of thymus-fed larvæ.

4. The inhibitory effect of the thymus is not due to a specific inhibiting substance in the thymus, but to the absence from the thymus of a substance required to develop the thyroid to the secretory state.

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