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3. Understand the Egg

Eggs, like the birds that lay them, come in many different shape and sizes. However, their basic structure is always the same.

There are 6 main parts to a fertile egg: the shell, the shell membrane, the albumen (egg white), the yolk (yellow), the chalazae (supporting tissue) and germinal disc (the fertile area visible as a white spot on the egg yolk).

The Shell

Shells can often vary in shape from species to species, however, usually one end is round and the other end is more pointed. This means that if an egg happens to roll, due to wind or other elements, it never rolls too far from the nest; a clever evolutionary feat of Mother Nature.

The shell of an egg is porous; meaning liquids and gasses can pass through it. The porous shell of the egg allows an easy interchange of oxygen, moisture and carbon dioxide.
Another fascinating feature of the shell is that it is thinker and denser on the outside than on the inside. This allows the chick to break out of the egg much easier than a predator could break in.

Egg Yolk

The yolk is, in essence, the food store of the egg and is made up of water, proteins, fats, vitamins and minerals. This food store is not primarily used in the incubation of the egg as many people think, but is the main source of food for the newborn chick within the first few days of hatching.

Day old chicks do not require feeding straight away after hatching, as they have an adequate amount of food available from the egg.


These are spirals of supporting tissue within the egg white, or albumen. The Chalazae are connected at both ends of the egg, holding the yolk in its central position.
The spiral threads are coiled in opposite directions, meaning it is vital not to turn the eggs continually in one direction; this will result in one of the threads becoming too tight, whilst the other becomes too slack. Eventually one will snap and the yolk will not be able to keep its position inside the egg.

During egg incubation, eggs should be turned through roughly 45 degrees in both directions; the chalazae will therefore remain intact.

Shell Membrane

There are two membranes inside the egg. The outer membrane lines the inside of the shell; whilst the inner membrane is only loosely appended, separated from the outer membrane at the broad end of the egg by the air sack (also known as the air cell). During incubation, the space between the two membranes increases as the egg looses moisture; this increases the air supply available to chick in the final few days before hatching.


Also known as the “white” of the egg, the albumen is composed of protein, water, vitamins and minerals. The albumen’s primary purpose is to protect the yolk from damage. However it is also a food store for the growing embryo during the egg’s incubation.

The albumen is composed of three different layers, each performing different function. The thin, watery outer layer has contact with the inner and outer membrane and allows the diffusion of gasses and moisture with the outside world.

The thick and viscous, jelly-like, inner layer provides a cushion for the delicate yolk, absorbing any shocks that the egg may suffer. The inner-most layer of the egg white is also thin and watery, acting as a lubricant for the egg yolk, allowing it to rotate freely in the middle of the egg. This ensures the yolk and germinal disc have ready contact with fresh supplies of food and oxygen.

Germinal Disc

The germinal disc is the fertile part of the hatching egg; the part where the male and female cell unite. A broody hen, or correctly calibrated egg incubator, the cells will begin to divide. Incubation and embryonic development have begun.

Nutrition of the egg

The egg is made up of many nutrients, vitamins and minerals that come from the parent. Hens need a variety of nutrients at least two weeks prior to laying.

The most important nutrients are:
  • Vitamin A – Obtained from green feeds and vegetables; a lack of Vitamin A causes poor hatch rates, weak chicks and poor resistance to disease.
  • Vitamin D – Obtained from sunlight; a lack of Vitamin D causes weak bones and misshapen eggs.
  • Vitamin E – Obtained from seed germs (i.e. Wheat); a lack of Vitamin E can result in weak chicks and circulatory failure.
  • Vitamin K – Found in most green feeds; a lack of Vitamin K can cause haemorrhaging.
How the egg is formed

The hen’s ovaries are positioned in the back of her abdominal cavity. There are several yolks inside the ovaries, at different stages of development. As they grow larger over time, the yolks move to the top of the oviduct passage, which eventually lead to the uterus.

In the upper-oviduct, the male sperm fuses with the female cell in the yolk. The fertilised yolk then travels down the oviduct (or egg canal) where it acquires coatings of albumen (egg white) followed by the shell membranes, the shell gland produces the shell and the completed egg travels to the uterus.

By the time the egg reaches the uterus, the hard shell has completely formed. It then moves to the vagina to be laid. This whole process usually takes 24 hours, however can sometimes take 36 hours.
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