As the embryo develops on the yolk, the yolk gets to be lighter and lighter. This causes it to drift upward within the egg. The yolk is held in place within the center of the egg whites by the chalaza (which serves as the axis to keep the fetus on top of the yolk). The yolk only attaches to the ends of the egg, not the sides. So, the yolk proceeds moving upward. If not turned for long periods the yolk will in the long run touch the inward shell films. When the developing life touches the shell layers, it'll adhere to the shell and die. Frequently turning the egg will prevent this, and guarantee sound embryo development. Another key advantage to turning your eggs is that by doing so, you're enabling the embryo to access new nutrients and oxygen within the egg. Turning moreover moves metabolic waste away from the developing embryo. Usually particularly critical in the first week of incubation
If you have an egg incubator:
Check and maintain the egg incubator's turning system before setting eggs. Failure in turning results into poor hatch rates.
Make sure that turning movement is smooth. Jolting affects hatchability and quality of chicks.
It is advisable not to turn eggs the first 12 hours in the setter especially if the fertilized eggs were transported from a different location. To restore their 'internal equilibrium,' eggs require some downtime.
After 15 days of incubation, turning eggs is no longer necessary.
It is good to leave the eggs in a horizontal position to promote enhanced air movement, especially in incubators with insufficient cooling capacity.