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Conclusions1. At sperm concentrations from 1 to 6 billion cells per cc an increase in concentration was accompanied by a decrease in sperm respiration rate. At concentrations of 0.2 to 1 billion sperm per cc this effect of concentration was not invariably observed. 2. In contrast with the behavior of sperm cells, respiration rates of yeast cells remained constant at concentrations from 1 to 8 billion cells per cc. 3. The effect of sperm concentration on sperm respiration rate does not appear to be due to changes in pH, diminished supply of O2 to individual cells, an enzyme, or products of metabolism. 4. When comparisons are to be made of relative respiration rates of semen samples, the samples must be adjusted to the same level of sperm concentration.

Conclusions1. Extra-cellular respiration was exhibited by seminal plasma of the boar. Oxygen consumption of seminal plasma ranged from 5 to 22% of that of the whole semen. The R.Q. was unity. After the plasma had been held for 5 minutes at 100°C respiration at reduced intensity was observed. Respiration was not exhibited by that fraction of the seminal plasma which passed through a porcelain filter. 2. The metabolic rate of the ejaculate of a cryptorchid boar was of the same order as that of seminal plasma. 3. No errors in measurement of oxygen consumption were detected which could be attributed to the presence of bacteria.

Conclusions(1) Hydrogen ion concentrations of the media in which sperm of the boar and ram are suspended definitely influence respiration rates of the sperm. (2) Optimum pH for respiration of boar semen is 7.2 to 7.3, and for ram semen 7.0 to 7.2. (3) As the pH is raised or lowered from the optimum the respiration rate progressively declines. (4) Unit change in pH has significantly less influence on respiration rate of ram sperm than on that of boar sperm. (5) Differences of 0.4 pH unit or more are accompanied by significant differences in respiration rates of boar and ram semen. Conditions for valid comparisons of different samples are presented.