Candled Eggs
When the unfertilized egg is candled, a shadow of the yolk is seen at the top of the egg (side view-dark area). If the egg is turned toward the large end, (angle view) the outline of the air space is visible. If the egg is moved on the candler light, the structures within will float around.
The Yolk
The formation of the egg starts in a hen's ovary. (Watch for more on the formation of an egg on Day 5.) What appears to be the yolk of an egg is actually a single cell also known as the ovum. The cell membrane of the ovum is surrounded by a non-cellular vitelline membrane produced by the ovum. The ovum stores nutrients in its cytoplasm as yolk granules for use by the embryo if the ovum is fertilized. (Watch for more on egg nutrition on Day 8.) There are two kinds of yolk: Yellow yolk is produced during the day and contains more fat. White yolk is produced at night and contains more protein. Most of the white yolk is located directly below the nucleus (the position of the possible future embryo) in the latebra and the nucleus of Pander. But as egg laying time grows closer,yolk is produced continually and begins to accumulate rapidly. For the last seven or eight days before the ovum is ovulated, yellow and white yolk are laid down in rings like those of a tree. When the ovum has accumulated enough yolk to grow a chick, it is released from the ovary.
Meiosis and Mitosis
Shortly after release from the ovary but before the outer layers of the egg are added, the nucleus undergoes meiosis to prepare it for fertilization. Only one of the cells produced during meiosis goes on to become the embryo while the rest disappear. The ovum now contains a haploid number of chromosomes and is ready to be fertilized by a sperm. If a sperm from a rooster is present, fertilization can take place. The resulting zygote begins simple cell division called mitosis. By the time the egg is laid, the embryo has reached the blastoderm stage. Whether the egg is fertilized or not, it will continue down the oviduct to be covered by albumen and a shell before it is laid. Fertilized eggs go on to become chicks; unfertilized eggs go on to become omelettes.
The Albumen
As the ovum travels down the oviduct to be laid, it is covered by albumen. The albumen is divided into an inner and outer layer by a fibrous layer. The inner layer usually becomes thicker near the yolk. The chalazae act to suspend the yolk in the albumen by anchoring it to the inner surface of the shell on either end of the egg (like a hammock). The inside of the shell is lined by a double-layered shell membrane. At the large end of the egg the double membrane separates to form an air pocket. If the temperature of the egg changes, or if some of the water in the egg evaporates,the air pocket becomes larger. Can you think of why this would occur?
The Shell
The egg shell is added last. (Why is this a good idea?) The shell is composed mostly of calcite, a crystalline form of calcium carbonate. The calcium needed to make a shell comes from the hen. During egg-laying periods, the hen stores calcium from her diet as extra bone in the marrow cavities of her long bones. This extra bone is referred to as medullary bone and occurs in female birds when they are actively laying eggs. Long bones include the femur (upper leg) and humerus (upper wing). When the shell is ready to be added to an egg, calcium is reabsorbed from the medullary bone into the hen's bloodstream and transported to a shell gland in the oviduct to be secreted as shell.