The shell is the eggs first line of defence against contamination and is largely composed of calcium carbonate. The shell is covered with a protective coating called a cuticle, which helps to preserve freshness.
The strength of the shell is greatly influenced by the minerals and vitamins in the hen's diet, particularly calcium, phosphorous and Vitamin D. Shell thickness is also dependent on egg size and a hen's age. As the age of the hen increases so does the size of the egg. As the egg becomes larger, the shell becomes thinner as it is stretched.
Thousands of tiny pores are scattered over the shell surface and as the egg ages the pores permit moisture and carbon dioxide to move out and air to move in to form the air cell. The cuticle blocks the pores and thereby retains the freshness of the egg and assist in the prevention of bacterial contamination.
Directly inside the shell are the inner and outer shell membranes, which are formed when an egg is laid and it begins to cool.
The air cell is found between the white and the shell at the large end of the egg. It is formed when the inner shell membrane separates from the outer shell membrane as the egg cools after being laid.
albumen (egg white)
The egg white accounts for most of the egg's liquid weight and contains more than 50% of the eggs total protein. It consists of 4 layers of thick and thin consistencies. As an egg ages the egg white tends to thin out as its protein changes in character.
The yolk makes up 33% of the liquid weight of the egg and contains all the fat in the egg and less than half of the protein. The yolk also contains a higher proportion of the egg's vitamins than the egg white, with all the egg's vitamin A, D & E in the yolk.
These are the strands of egg white that anchor the yolk in the center of the thick white. The more developed the chalazae, the fresher the egg.
The germinal disc is barely visible as a slight depression on the surface of the yolk. When the egg is fertilised, of which none of our eggs are, the sperm enter by way of the germinal disc.
An egg's weight is divided in the following proportions: 11% shell, 58% white and 31% yolk.