The remainder of the egg i.e. the albumen, the shell membranes and the shell are produced by different segments of the oviduct. These segments are:

• Infundibulum
• Magnum or ampulla
• Isthmus
• Uterus or shell gland
• Vagina
• Cloaca

In the egg laying hen the oviduct is a tube like organ consisting of the previously named segments with one end lying adjacent to the ovary and the other entering the vent. It is approximately 70 centimetres long and is very glandular. The glands of the different segments produce the remaining different parts of the egg. Because of its function the oviduct is very well supplied with blood vessels.

Infundibulum

This segment is funnel shaped and lies adjacent to the ovary. It is up to 9 centimetres long in the laying hen and has the function of searching for and engulfing the yolk that has just been released from the follicle into the adjacent ovarian pocket or body cavity. The yolk remains in the infundibulum for about 15 minutes and it is in the infundibulum that fertilisation takes place.

If the infundibulum should malfunction and not engulf the yolk, the yolk will remain in the ovarian pocket from where normally they will be absorbed within three days. If the number of such occurrences reaches a high level, the yolks will accumulate in the ovarian pocket faster than they can be absorbed. Such birds’ are called internal layers - the abdomen becomes distended and the hens adopt a very upright stance.

Magnum or ampulla

The magnum is the longest segment at up to 40 centimetres long. Its function is to add approximately 40% of the albumen to the developing egg that takes about three hours to move through. These percentages vary considerably depending on factors including the genetics of the hen, age of the bird, the egg’s age and/or storage conditions. However, in a good quality, freshly laid egg the above relationship mostly applies.

There are four different types of albumen in the normal egg:

The chalazae are two twisted chords of albumen extending from the opposite sides of the yolk into the remaining albumen in the broken out egg. 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 different layers - 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.

Isthmus

The isthmus is approximately 12 centimetres long and has the functions of adding approximately 20% of the albumen and the shell membranes to the egg. There are two shell membranes:

1. The inner shell membrane - laid down first
2. 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. While the egg is still in the oviduct the shell membranes appear as one over the total surface of the egg so close are they associated with each other. 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 1.5 centimetres in diameter and approximately 0.5 centimetres deep.

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 the penetration into the egg by bacteria and fungi. 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. The developing egg remains in the uterus for 18-20 hours while approximately 40% of the albumen and all of the shells are added. 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 - the more grains deposited in the isthmus, the thicker will be the final shell.

The shell of an egg is formed in two layers:

1. Mammillary layer: a sponge like layer composed of soft calcite crystals (CaCO3). This layer is the inner layer.
2. Palisade layer: formed of columns of hard calcite crystals - the longer the columns the stronger the shell. This layer is the outer layer.

The calcium for the eggshell comes from three sources - the diet, special bone called medullary bone (found in the cavity of long bones) of point of lay pullets and the skeleton. The hen uses approximately 2.5 grams of calcium in the formation of one normal egg. She cannot absorb sufficient calcium from her diet each day (approximately 2.0 grams per day) to supply this need and hence, it becomes necessary for her to utilise skeletal calcium to make up the shortfall. 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.

The carbonate ions go with the calcium to form the calcium carbonate of the egg’s shell come from the blood and from the shell gland. 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.