1. A buff-coloured precipitate is obtained by the addition of N/20 iodine solution to dilute acetic acid extracts of fresh anterior lobe substance. The volume of the precipitate depends upon the volume of the iodine solution added. Similar acetic acid extracts of tissues and extracts of the anterior lobes prepared with other extracting media do not give the same precipitate with iodine.

2. The precipitate indicates the presence and, within limits, gives an approximate measure of the metamorphic factor in fresh gland extracts of the anterior lobe.

3. Dilute acetic acid extracts of the precipitates after removal of the iodine by shaking with alcohol are biologically active and characterised by their phosphate content.

4. Similar extracts of the posterior lobe give an iodine precipitate but the phosphate content is much lower than that of the anterior lobe precipitate.

5. The iodine in these precipitates is adsorbed or loosely combined. The adsorbed iodine is chemically more reactive than dissolved iodine or iodine crystals giving rapid evolution of nitrogen from sodium azide compared with the failure of the others.

6. The biological activity of anterior lobe extracts varies with the time of extraction and the concentration of the extracts. There is a maximum for the time of extraction at any particular concentration and acid strength after which a loss of potency occurs. There is a continuous extraction--rapid at first--of the melanophore stimulant. The amount increases with the period of extraction and concentration, but is independent of the strength of the acid used in extraction.

7. The metamorphic factor is slowly destroyed by pepsin but rapidly by trypsin.

8. The biological activity is diminished by boiling, or by boiling with dilute hydrochloric acid, sodium hydroxide, or sodium carbonate.

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