Nearly all animals produce eggs, but only some of them lay eggs outside of the body. Birds' egg shells are one of nature's great design solutions: thin, porous, yet amazingly strong. They are shaped and structured to provide protection and nutrients for the developing embryo.

The structure of a hen's egg.

Birds' eggs have five important parts. The most important is the germinal disc. This is found inside the yolk and is the cell nucleus from which the young bird would have developed if the egg had been fertilised. Commercially produced eggs for eating are never fertilised.

The bulk of an egg consists of a thin white solution of proteins, mineral elements, carbohydrate and water called albumen. In hens' eggs this makes up 58% of the total egg mass. Egg yolk makes up the remaining 31% and is held in place by two dense cord-like strands which are called chalazae.

The shell of an egg is made from a type of calcium carbonate called calcite. This is the same material that also occurs in marble, limestone, coral and chalk! Eggs shells vary naturally in colour, depending on the breed and age of the hen.

Inside this thin shell are two semi-permeable membranes. These act as filters to protect the egg's contents from dirt and bacteria. The outer membrane is the one upon which the shell is built. The inner membrane surrounds the white and the yolk. A third membrane, the vitelline membrane, supports the yolk.

(a) Print out the above diagram: 'The structure of a hen's' egg and label it correctly. (b) Draw a pie chart to show the mass of an egg, showing the percentages of shell, albumen, and yolk. (c) Write down the five important parts of an egg and their functions. You may want to incorporate this information into your diagram: 'The structure of a hen's egg'. If so, you will need to work out the best way to present this. (d) From what you now know, work out the chemical formula for calcite.

1. Egg yolk contains fat which is present as an oil-in-water emulsion. (i) What is an emulsion? (ii) List three other emulsions that you might find in your kitchen. 2. Name five animals that lay eggs outside of the body. Your list must include: an amphibian, a reptile, an insect and two mammals. Draw each of the animals and state clearly which animal group they belong to. 3. Why might the colour of an egg shell depend on the breed of the hen that laid the egg?