Eggs are a unique product of nature. They are among nature's most nutritional and perfectly packaged foods. As a source of high quality protein, they are classified as being alternatives to meats. Eggs also contribute cooking characteristics that cannot be provided by any other food. Eggs are also among the most economical foods and are available in all supermarkets. Therefore, eggs are ideal as a food when nutritional and economic values, versatility, and availability are considered.
Each egg is composed of internal yolk and albumen contents sealed within a seamless, hard shell. The calcium carbonate shell makes up about 11% of total egg weight while the yolk and albumen form 31% and 58%, respectively. The proportions of these egg constituents remain relatively constant regardless of the egg's quality or size.
Egg shell color does not affect the quality, taste, nutritional value, or cooking quality of eggs. The shell color is determined by the breed of hen laying the egg and cannot be altered. Yolk color, however, can be altered by changes in the composition of the hen's feed. Yolk color has no effect on nutritive value.
Blood spots in the albumen do not change the flavor or nutritive value, although they are unsightly and repulsive. They can be easily removed prior to cooking by using a spoon or pointed knife. These spots do not indicate whether the egg is fertile or not. They form when a blood vessel in the oviduct of the hen ruptures during the egg formation process. Only about 1% of all eggs contain blood spots and most are removed during the egg grading process.
White, stringy objects are often observed in the albumen of some eggs. Many persons think that these are meat spots or that they indicate a fertile egg. Neither assumption is true! These structures are called chalazae (pronounced "ka-lay-zee") and are a natural portion of the thick albumen of a fresh egg. These strands function as anchors to hold the yolk centered in the egg. Their presence is desirable in that only the freshest eggs have chalazae that disappear after extended storage or improper storage conditions.
Source: Mississippi State University Extension Service: http://www.msstate.edu/dept/poultry/4heggs.htm