1. The effects of external osmotic pressure on the rates of development and emergence, respiration, and on changes in glycogen, glycerol, and trehalose concentrations have been studied in cysts of Artemia salina.

2. The only measured effect of external osmotic pressures less than 30 atm. on emergence and development is to determine the time required for the embryo to emerge from the cyst. Above this value the onset, rate, and final percent of emergence decrease. No emergence occurs at osmotic pressures greater than about 65 atm.

3. The oxygen consumption decreases with increased osmotic pressure, and is negligible at about 65 atm.

4. Several lines of evidence show that trehalose is the respiratory substrate, that most of the trehalose present in the dormant embryo is converted to glycogen and glycerol during development, and that the direction and extent of these conversions are controlled by the external osmotic pressure.

5. Glycerol appears to be present in at least two distinct locations in the cyst: within the embryo, and between the embryo and shell. Glycerol in the latter location is released into the medium at the time of emergence; the embryonic glycerol is rapidly metabolized after emergence.

6. The physiological significance of glycerol and trehalose in the emergence process is discussed.

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