1. Unfertilized eggs of Chaetopterus consume about 2.4 mm.3 O2 per hour per 10 mm.3 eggs at 21°C.

2. In the 1st hour after fertilization, the fertilized eggs consume oxygen at about 53 or 54 per cent of this rate, which is about 1.3 mm.3 O2 per hour per 10 mm.3 eggs at 21°C.

3. For the first 6 hours after fertilization, at 21°C., the curve of the rate of oxygen consumption is slightly asymmetrically sigmoid. The prefertilization rate is regained between 4½ and 5 hours after fertilization. Soon after 6 hours, ciliary activity begins, and the rate of oxygen consumption rises rapidly.

4. The unfertilized eggs of Arbacia punctulata consume about 0.36–0.5 mm.3 O2 per hour per 10 mm.3 eggs at 21°C. The absolute determination is difficult as these eggs are highly sensitive to shaking in the manometer vessels, and these difficulties are discussed.

5. The fertilized eggs of Arbacia punctulata consume oxygen at the rate of about 2.0 mm.3 O2 per hour per 10 mm.3 21°C. At 1 hour after fertilization the rate is already rising.

6. A comparison of the absolute rates of oxygen consumption, and the changes in rate at fertilization of these and a number of other eggs, together with a theoretical discussion, and a discussion of discrepancies in measurements on the eggs of Arbacia punctulata, is contained in the fifth paper of this series (21).

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