Eggs of plaice, Pleuronectes platessa L., were incubated in salinities of 5, 17.5, 35 and 50%. After fertilization a large concentration difference is maintained between the yolk and the external medium; this difference is present throughout development. The unfertilized egg becomes isosmotic with the medium within 24 h.
High mortalities were found at all stages of development in salinities below 17.5% most eggs died in the blastula or early gastrula stages. In addition, mechanical disturbance (e.g. during transport) also led to many deaths at this stage.
Water movement could occur between the environment and both the yolk and embryonic cells, it was detected by changes in the freezing-point of the yolk and changes in the volume of the blastula cells.
After hatching, the yolk-sac larvae tolerated salinities of 15–60% for 1 week, and the body fluid concentration was regulated within fairly narrow limits in these salinities. After metamorphosis, tolerance to high salinities was less (45% for 1 week), but tolerance and regulation in low salinities was greater (2.5% for 1 week). It is suggested that functional changes in the epidermis and kidney underlie these changes in tolerance.
An investigation of the effects of salinity on the early development of the herring (Holliday & Jones, 1965) suggested that the ability to osmoregulate was present in the cells of the early blastula, but not in the vitelline membrane of the fertilized egg. The yolk was not fully regulated until gastrulation was complete; until this time it was approximately isosmotic with the medium in which it was being incubated.