Abstract
The paper reviews experiments conducted to find out how teleost embryos maintain a proper water and solute status during the stage from spawning until osmoregulatory organs have been developed. The yolk osmolality is influenced but little by the osmolality of the ambient medium. Osmotic volume changes and uptake of excess sodium chloride are limited by extremely low permeabilities of the egg cell membrane and embryonic ectoderm. The low permeabilities may be due to an unusual composition of the cell membrane. There is indirect evidence for cod embryos being able actively to absorb water against the osmotic gradient. In most marine species a decrease in the potassium content counteracts the osmotic effect of the volume decrease and the increase in sodium and chloride. In gadoid eggs a decrease in the content of free amino acids outweighs the potassium decrease, simultaneously supporting the energy production of the embryo. Indirect evidence supports the view that the low permeabilities to water and dissolved salts do not conflict with the need of a higher oxygen permeability.