Wet mass and water content of four lots of whole eggs did not change throughout embryonic development of rainbow trout Oncorhynchus mykiss. Eggs in all four lots accumulated Na + . Eggs in lots 2 and 4 also accumulated Ca 2+ and Cl , whereas eggs in lot 1 showed no significant change in Ca 2+ or Cl , and eggs in lot 3 showed no change in Cl and a small loss of Ca 2+ . Although the Na + content of embryonic tissues increases in the later stages of development, the yolk sac content remained constant, indicating uptake of Na + from the environment. Na + uptake by whole eggs was non-saturable, consistent with diffusion of Na + across the chorion into the perivitelline fluid. Na + uptake in dechorionated embryos was saturable, as was Ca 2+ uptake by both whole eggs and dechorionated embryos, consistent with active uptake or facilitated diffusion mechanisms at the surface of embryos. Very low Ca 2+ uptake rates in dechorionated embryos suggest that the Ca 2+ uptake mechanism is not fully developed until after hatching. 2001 The Fisheries Society of the British Isles
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