1. In the presence of thyroxine in concentrations of 1/45,000 to 1/75,000 the initial rate of oxygen consumption of the spermatozoa of Echinus miliaris is the same as in sea-water without the drug, but this rate of oxygen consumption is prolonged for a longer time in the presence of the drug. The rate of oxygen consumption in the presence of the drug falls after this period, and at the end of 2-4 hours is approximately equal to that in clean sea-water.

2. In a concentration of 1/30,000 the drug is toxic, and of 1/100,000 the effect is incomplete.

3. These effects are parallel to, and apparently of the same nature as, the effects of secretions of the eggs of the same species upon the oxygen consumption; but the effects of the egg secretions are much greater in extent.

4. The suggestion is made that the egg secretions contain, in addition to their other known constituents, either a body of which the physiological effect is similar to that of thyroxine, and which is perhaps similar to it in chemical structure, or some constituent from which such a body is built, when it is absorbed by the spermatozoon.

5. In its action upon the spermatozoon the effect of thyroxine is to remove or reduce the cause of the decreasing activity of the spermatozoon in sea-water (as revealed by its oxygen consumption). This effect is immediate and may take place at any period during the life of the spermatozoon at which its oxygen consumption in the presence of thyroxine is greater than that in sea-water.

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