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Formation of sub-embryonic fluid (SEF) is a key aspect of the physiology of the early avian embryo. Here we review the process of SEF formation and the factors which influence its composition and the rate of SEF production and depletion in the Japanese quail and domestic fowl. There is particular emphasis on the role of turning of the egg during incubation and we briefly consider the broader role of egg turning during avian incubation. The bulk of the review deals with the growth of the area vasculosa of the yolk sac membrane, the cellular processes of SEF formation, and the water and electrolyte physiology of the avian embryo during the first half of incubation.We conclude with a brief discussion of the areas for future investigation.

1. The effects of egg turning and fertility upon sodium concentration of albumen of the Japanese quail is described for up to 72 h incubation. 2. For incubated eggs the sodium concentration of albumen adjacent to the yolk sac was lower than that from albumen next to the shell. Static incubation increased the magnitude of this difference, such that albumen adjacent to the yolk sac was substantially depleted of sodium. This was found at the yolk equator and the yolk vegetal pole of both fertilised and unfertilised eggs. 3. Unincubated eggs also had a lower sodium concentration of albumen adjacent to the yolk sac compared with albumen next to the shell. 4. After 48 h of incubation yolk sodium concentration was substantially lower than albumen sodium concentration in both fertilised and unfertilised eggs, whether eggs were turned or not. 5. It is concluded than in unturned eggs the depletion of sodium from albumen adjacent to the vitelline membrane is not produced by ion transport processes but results from a passive movement of sodium into the yolk. Egg turning reduces the magnitude of the depletion of sodium from the albumen adjacent to the yolk sac by stirring the albumen, so permitting the full expression of ion and water transport across the blastoderm into the yolk sac.

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