The structure of a chicken egg
Below is a breakaway diagram of a chicken egg:
Here are brief explanations of each part:
Shell – The outer eggshell is made almost entirely of calcium and has as many as 8,000 tiny pores. It is semi-permeable, so it lets gas exchange occur, but keeps other substances from entering the egg. The thickness of an eggshell is determined by the amount of time it spends in the shell gland (uterus) and the rate of calcium deposition during egg shell formation.
Inner and outer shell membrane – These two membranes, the inner and outer, are just inside the shell surrounding the albumen (white). The two membranes provide an efficient defense against bacterial invasion and are made partly of keratin. The outer membrane sticks to the egg shell while the inner membrane sticks to the albumen. When an egg is first laid, it is warm. As it cools, the contents contract and the inner shell membrane separates from the outer shell membrane to form the air cell.
Chalazae – Are twisted in opposite directions and serve to keep the yolk centered. The more prominent the chalazae, the fresher the egg.
Outer thin albumen – The outer thin albumen is a narrow fluid layer next to the shell membrane. It is the watery part of the egg white which is located farthest from the yolk
Inner thick albumen – The inner thick white (chalaziferous layer) is a dense, matted, fibrous capsule of albumen around the membrane located nearest the yolk. This part of the egg is a excellent source of riboflavin and protein. In high-quality eggs, the inner thick albumen stands higher and spreads less than thin white. In low-quality eggs, it appears thin white.
Yolk membrane – The clear casing that encloses the egg yolk.
Germinal disk – A small, circular, white spot on the surface of the yolk is where the sperm enters the egg. The embryo develops from this disk, and gradually sends blood vessels into the yolk to use it for nutrition as the embryo develops.
Yolk – The yellow yolk is a major source of vitamins, minerals, almost half of the protein, and all of the fat and cholesterol. The yolk contains less water and more protein than the white, some fat, and most of the vitamins and minerals of the egg. These include iron, vitamin A, vitamin D, phosphorus, calcium, thiamine, and riboflavin. The yolk is also a source of lecithin, an effective emulsifier. Yolk color ranges from just a hint of yellow to a magnificent deep orange, according to the feed and breed of the hen. The yolk contains the food that will nourish the embryo as it grows.
Chalaziferous albumen – Is a dense, matted, fibrous capsule of albumen around the membrane of the yolk. The matted fibrous capsule terminates on each end in the chalazae.
Chalazae – The spiral, rope-like strands that are twisted in opposite directions and keep the yolk centered. The more prominent the chalazae, the fresher the egg.
Air cell – An air space forms when the contents of the egg cool and contract after the egg is laid. The air cell usually rests between the outer and inner membranes at the egg’s larger end. As the egg ages, moisture and carbon dioxide leave through the pores of the shell, air enters to replace them and the air cell becomes larger.
Cuticle (also called bloom) – The shell is produced by the shell gland (uterus) of the oviduct, and has an outer coating called the bloom or cuticle. When the cuticle or bloom is deposited by the hen on the shell this acts as a barrier to keep bacteria from entering the egg. These barriers provide a good line of defense against invading bacteria. Most of the protective covering is removed from eggs when they are mechanically washed.
To see embryos in various stages of development that were taken from a hen, check out Unformed eggs- A look inside the hen
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