The eggshell is a hard, three-layered container composed of calcium carbonate. Its purpose is to protect the enclosed embryo from the weight of the parent's body while the bird develops inside. With the exception of those birds that warm their eggs in mounds of dirt, all birds use body heat to incubate their eggs and turn them regularly to keep the temperature even. In natural incubation the hen turns the eggs as she gets off the nest and moves them with her beak while she is sitting on them.
Like all living things, the developing chick must have three things to live—food, water and air. The food is provided by the yolk, which is mostly protein. Protein helps to build strong bones and muscles. Water comes from both the yolk and the albumen. The albumen is the clear portion of the egg, most commonly called the "egg white". The albumen is 85 percent water, and the yolk is about 50 percent water. Air passes through the shell and the membrane. The chicken uses the oxygen and passes carbon dioxide back through the shell.
Each egg shell has a coating or covering, called a bloom, that seals its pores, prevents bacteria from getting inside and reduces moisture loss. Eggs are washed before they are sent to the market. This is necessary for cleanliness but removes the bloom. To restore this protection, packers give the eggs a light coating of edible mineral oil. Properly handled and stored, eggs rarely spoil but will simply dry up if kept long enough.
Although the exact shape of an egg is as individual as the hen herself, most eggs are roughly egg-shaped. Some abnormalities that can be found in the shells of fresh eggs are ridges, bulges and rough texture. Eggs having any of these abnormalities would get poor eggshell ratings from the US Department of Agriculture (USDA). Some extreme oddities in eggshell shapes include shells that are long, appear bent, or look as though they have been mashed in on one side. It would be very rare to find any of these shapes on grocery store shelves. Wild birds have shells that are more pointed than those of most domesticated varieties.
The size of an egg is an inherited characteristic, and poultry breeders spend large amounts of time and effort to select strains for egg size. When a pullet (a young hen less than a year old) first begins laying eggs, her eggs are small. After 15 to 20 days of laying, the size of her eggs will reach the size of a standard grocery store egg. Egg size varies greatly from one kind of bird to another. The eggs of domestic chickens weigh an average 58 grams. Those of domestic turkey are about 85 grams. Hummingbirds lay eggs weighing a half a gram. Quail eggs weigh an average nine grams, and ostriches lay eggs weighing an average 1,400 grams.
- Plastic Egg Genetics
Genetics activity with plastic Easter eggs from Acess Excellence: The National Health Museum
Ryegrass Easter Baskets
Try planting baskets with real grass. Oklahoma is number one in the nation in the production of rye grass, a cool season grass that grows very quickly. Line your Easter baskets with plastic, and put in potting medium. Sprinkle rye grass seed on the surface, and spritz to moisten. Grass should begin to grow within a few days. You may also use wheat seed, which is available as wheat berries from health food stores.
Eggshell Grass Heads
Plant ryegrass in eggshells. Make collars from paper for the eggs to rest in. Use markers to make faces on the eggshells.
- Set up 60-70 "nests" of straw or paper bags with tops rolled down.
- Place one egg (or substitute, such as ping pong balls) in each nest (5 dozen total).
- Group students into five groups, and provide one empty egg carton for each group.
- As a relay, students will collect eggs and place them in the egg cartons. One student from each group collects at a time. First group to fill their carton wins.
- Write a story about an egg.
- Write about the most unusual place you have found or hidden an Easter egg.
The extraordinary strength of the eggshell inspired one of the most beautiful architectural forms in the world—dome construction. With dome construction, weight is distributed evenly around a central point, like the large end, or air cell end, of an egg.
St. Peter's Basilica in Rome is one of the oldest and most famous examples of dome construction. Other famous domed buildings include the US Capital in Washington, DC, and the Palazzo dello Sport in Rome, Italy, designed for the 1960 Olympic Games.
Test the strength of the dome with this lesson using eggs.