Outer liquid layer These two cords extend into the ends of the egg along the longitudinal axis and are parts of a very thin envelope of special albumen that surrounds the yolk and holds it in its position. The yolk has to remain centrally located for the survival of the embryo. The yolk turning or rotating as it passes along the oviduct causes the twisted effect of the chalazae. While the bird produces only dense albumen, as the egg moves along the oviduct, water is added thus making liquid albumen.
The rotation of the developing egg causes the albumen to separate into the inner liquid and the dense layers. The outer liquid layer is caused by the addition of more water when in the uterus. The dense layer contains significant amounts of mucin that binds it together in a jelly like form. As an egg stales, the amount of dense albumen decreases as it changes to the liquid form.
The liquid form increases in volume and becomes even more fluid. There are two shell membranes: The inner shell membrane — laid down first The outer shell membrane — laid down last and about three times the thickness of the inner membrane The isthmus takes approximately 75 minutes to carry out its tasks. However, as the egg cools after it has been laid, the membranes separate, usually at the larger end to form the air cell. The air cell in the new laid egg is approximately 1.
As the egg ages, the interior contents lose water and the air cell increases in size. This change in size is an indicator of egg quality as related to the age of the egg and the holding conditions. The shell membranes consist of a fibrous protein material and act as a barrier to bacteria and fungi penetration into the egg.
They also help reduce the rate of evaporation of water from the egg thus slowing the rate of deterioration of the egg. The isthmus also lays down the foundation for the shell by forming the first crystals of calcium carbonate on the outer shell membrane.
Uterus shell gland and eggshell quality The uterus is a relatively short, bulbous gland up to 12 centimetres in length. It is for this reason that the organ is often called the shell gland. Shell formation really begins by the deposition of small clusters of calcium carbonate crystals onto the outer shell membrane while in the isthmus. These are the initiation grains for the subsequent calcium carbonate deposition in the uterus.
The number of these grains is genetically controlled and is related to the subsequent shell thickness as the more grains deposited in the isthmus, the thicker will be the final shell. The shell of an egg is formed in two layers: Mammillary layer — a sponge like layer composed of soft calcite crystals CaCO3. This layer is the inner layer. Palisade layer — formed of columns of hard calcite crystals; the longer the columns the stronger the shell.
This layer is the outer layer of the egg. The calcium for the eggshell comes from the diet, a special bone called medullary bone found in the cavity of long bones and the skeleton.
The hen uses approximately 2. She cannot absorb sufficient calcium from her diet each day approximately 2. This is particularly so at night when most of the shell is being formed but the hen in unlikely to be eating.
In addition to the calcite, the shell also contains small quantities of sodium, potassium and magnesium. If anything should interrupt the supply of carbonate, thin-shelled eggs will result. This occurs in hot weather when hens pant to remove excess heat energy.
The increased respiratory rate removes carbon dioxide from the blood thus reducing the carbonate ions available for eggshell formation. Carbonic anhydrase is the enzyme which catalyses the conversion of carbon dioxide and water into carbonate ions. Zinc is the co-enzyme of carbonic anhydrase and any conditions resulting in Zn deficiency can lead to problems associated with egg shell formation. There are many factors that influence eggshell quality: Length of time in lay: The longer the bird is in lay, the weaker the shells will become because of her inability to obtain enough daily calcium from her diet to supply all of her needs for one egg.
As a consequence, better layers will deplete their skeleton calcium supply. This results in reduced food consumption and calcium and the reduction of carbonate ions because of panting. Eggs laid early in the morning are more likely to have thinner shells than those laid by the same bird later in the day. This is because in the case of those eggs laid early the shells have been deposited during the hours of darkness when the bird does not eat, and therefore no dietary calcium for the shell formation.
Stressed birds lay thinner shelled eggs. Body checked and misshapen eggs: Most of these defects are caused by the birds being startled shortly after the egg has entered the uterus and the first layers of calcium carbonate have been deposited. Certain diseases can cause weak shell and misshapen eggs.
Certain drugs influence eggshell formation and deposition. The shell of an egg contains openings or pores. The function of these pores is to provide for the gaseous exchange during incubation and embryonic development. The developing embryo requires oxygen and gives off carbon dioxide. When the egg is first laid most of the pores are closed. However, as the egg ages more and more pores open up.
The cuticle deposited on the outer shell is composed of organic material and water and blocks the pores. Vagina The vagina is about 12 centimetres in length. While not known for sure, it may have the function of adding pigment to the outer shell to provide the egg with its colour. Cloaca The egg is held in the cloaca immediately prior to being laid.
It may be in the cloaca for several hours, but usually is held there for a much shorter time. Although the egg usually enters this organ small end first, it usually rotates there to be laid by the large end first.
However, if the bird should be startled at this time the egg may be forcibly expelled small end first. The formation of an egg is a very complex activity during which much can go wrong.
The quality of the final product, the egg as it is laid, is influenced by both genetic and management factors.