I had the same thing in one of mine last weekend! not to worry. here's what it probably is.
Blood spots
Description
Blood spots vary from barely distinguishable spots on the surface of the yolk to heavy blood contamination throughout the yolk. Occasionally blood may be diffused through the albumen or white of the egg.
Meat spots
Description
Most meat spots are pieces of tissue from body organs, but some may be partially broken-down blood spots. They are usually brown in colour, and found in the thick albumen, chalazae, or the yolk. They range in size from 0.5 mm to more than 3 mm in diameter.
Incidence
The incidence of meat spots ranges from less than 3% to 30% or more. It varies with the strain of bird, increases with the age of bird and may be higher in brown eggs. Many meat spots are too small to be detected by candling, especially in brown eggs. Less than 1% of eggs are usually downgraded because of meat spots.