| Summary: | The yolk osmolarity of cod eggs decreases from about 400 mOsm 2 days after spawning to 300 mOsm at hatching, as opposed to a sea water osmolarity of c. l000 nOsm. The osmolarity difference forces an osmotic water loss upon the eggs. The embryos take 3-4 weeks to reach a developmental stage allowing them to compensate water losses by the mechanisms known from the adult fish. In this period the rate of osmotic water loss is minimized by a low water permeability: 0.24 x 10^-6 cm^3 cm^-2 S^-1 on day one to 0.6 X 10^-6 higher at hatching. Water content predicted from osmolarities and water permeabilities agrees well with the observed water content until the completion of epiboly, indicating that water balance is governed mainly by the passive, osmotic water loss during this period. The rate of osmotic water loss increases with temperature in nearly the same way as does the rate of embryonic development, thus the total water loss till completion of epiboly may be independent of the water temperature. At later developmental stages the observed water content gradually becomes much larger than predicted, indicating that the water balance after epiboly is not governed exclusively by the passive osmotic water loss.
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