No abstract

No abstract

In early stages of avian development, respiratory gas exchange takes place mainly through the blood vessels of the yolk sac membrane. Therefore, a short distance between the yolk sac membrane and the eggshell is required to ensure a sufficient oxygen supply of the embryo. In freshly laid eggs the yolk sphere is located approximately in the center of the egg, but soon after the beginning of incubation it moves to the upper pole right beneath the eggshell. The cause for this displacement is not known. To quantify the forces acting on the yolk sphere of chicken eggs during incubation, we measured the densities of albumen, yolk and subembryonic fluid and the viscosity of the liquid albumen. We found that between day 0 and day 4 of incubation the calculated buoyancy due to density differences between the egg components increases only threefold. This is probably too small to overcome the yield stress of the gel-like thick albumen that forms a capsule around the yolk sphere. A different process responsible for the yolk displacement is proposed. Under reduced gravity, the movements of the yolk are influenced substantially by the fluid properties of the thin albumen, especially yield stress. Furthermore, acceleration peaks present in a space laboratory will probably displace the yolk sphere away from the eggshell. This is strong evidence that an early chick embryo will not be able to survive in a space laboratory in ovo.