1. The respiratory quotient of sea-urchin spermatozoa has been determined with Pseudocentrotus depressus, Anthocidaris crassispina and Hemicentrotus pulcherrimus.

2. The R.Q. of sea-urchin spermatozoa, measured by the Warburg direct method, has been reported to be near unity. This was also the case with the present material when the suspending medium was sea water, the R.Q. being 0.8-1.0. It was found, however, that the pH of sperm suspensions was markedly different in the presence and absence of alkali to absorb C02.

3. When the pH of the suspension was fixed by such buffers as 0.025 M-glycyl glycine, the R.Q. measured by the above method was about 0.7. This is in accord with the results of earlier metabolic studies, which indicated that endogenous phospholipids are the main substrates for the respiration of sea-urchin spermatozoa.

4. The O2 uptake of the present material, however, was found to be little affected by variation in pH. The difference in the R.Q. values obtained with ordinary sea water and buffered sea water, therefore, cannot be explained in terms of pH.

5. When the spermatozoa were suspended in ordinary sea water, the utilization of endogenous phospholipids was much reduced in the absence of alkali, while in buffered sea water the change in phospholipids was almost the same, with and without the absorption of CO2.

6. Determination of the R.Q. by the first method of Dickens & Šimer, in which the O2 uptake and the CO2 output were measured with one and the same sperm suspension, gave a value of about 0.7 with both ordinary and buffered sea water.

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