A bird’s egg is a complex structure that houses the growing embryo and contains all the nutrition that is needed before the chick is ready to hatch.
It consists of four basic parts; the shell, the white, the yolk, and the germinal disc, which contains the DNA nucleus of the female cell, or ova.
Birds’ eggshells generally have a grainy texture and are covered in tens of thousands of pores. Eggshell is made primarily from calcium carbonate crystals stabilized by a protein matrix. The protein stops the crystal structure from being too brittle to keep its form, and the calcium causes the cortical reaction which starts embryogenesis
Eggshell is a semipermeable membrane, which means air and moisture can pass through its pores.
The germinal disc, also called the blastodisc, is a small, white, circular spot of cytoplasm that sits on the surface of the yolk.
It is a single layer of cells from which the embryo forms, and is composed of two layers; the hypoblast which lies adjacent to the blastocyst cavity, and the epiblast, which lies adjacent to the amniotic cavity.
The vitelline membrane is a clear casing that encloses the yolk. It is made mostly of protein fibres and has protein receptors that are needed for sperm recognition and binding.
It consists of an inner layer which is laid down in the ovary, and an outer layer which is secreted in the oviduct.
The primary function of the yolk is to supply nutrients to the developing embryo. It is spherical in shape and is suspended in the egg white. It contains less water and more protein than the white, fat and cholesterol, and most of the vitamins and minerals of the egg, including iron, vitamin A, vitamin D, thiamine, phosphorus, calcium, thiamine, and riboflavin. The proteins function partly as foods in their own right and partly in regulating the storage and supply of the other nutrients.
Birds’ egg yolks are usually yellow in colour ranging from just a hint of yellow to a deep orange depending on the species and diet of the bird. The more fat the yolk contains the darker the colour.
If ovulation occurs too rapidly an egg may be formed with a double yolk. Double-yolked eggs will rarely produce chicks.
The chalazae are two spiral strands of tissue that hold the yolk in the centre of the egg. Each chalaza contains fibres that are identical to the fibres of the vitelline membrane, except they are coiled and embedded in the albumen.
If the egg is placed in an upright position the chalazae will move so that the yolk rotates and the cytoplasm stays at the uppermost side.
The albumen is the egg white and makes up about a third of the egg by weight. 90% of this is water with proteins, minerals, fat, and glucose making up the remainder. The albumen also contains globulins which help provide immunity from disease.
It is made up of alternating thick and thin layers formed from secretions of the anterior section of the oviduct.
A thick layer of albumen is in direct contact with the surface of the yolk surrounded by a watery layer. This is followed by another more viscous layer, and finally another watery layer is in contact with the shell’s inner membrane.
The albumen protects the yolk and provides additional nutrition for the embryo. When the chick is ready to hatch it acts as a lubricant to help the chick turn and push itself out of the egg.
After an egg is laid the contents cool and contract and the inner shell membrane separates from the outer shell membrane forming an air cell.
As the egg ages, moisture and carbon dioxide leave through the pores of the eggshell, air enters to replace it, and the air cell enlarges.
The cuticle or bloom is a thin protein membrane that covers the eggshell and seals the pores while still allowing air to get inside for the growing embryo. The cuticle reduces moisture loss and prevents bacteria from entering the egg. It is the first line of defence against infection.
The allantois is a sac-like structure filled with clear fluid that is involved in nutrition and excretion. It is a reservoir for nitrogen waste and supplies oxygen to the embryo, absorbed through the eggshell.
The amnion which surrounds and protects the embryo is filled with a clear, transparent liquid which provides the embryo with a stable fluid environment and provides a cushion against bumps and knocks.
The amnion is lined with ectoderm and covered with mesoderm germ layers. It also helps prevent against fluid loss from the embryo.
