Eggshell

The intact shell of a white chicken egg

The shell of a brown chicken egg

An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.

The generalized eggshell structure, which varies widely among species, is a protein matrix lined with mineral crystals, usually of a calcium compound such as calcium carbonate. It is calcium build-up and is not made of cells. Harder eggs are more mineralized than softer eggs.

Diversity

Some other colors of chicken eggs

Insect eggs

Insects and other arthropods lay a variety of styles and shapes of eggs. Some have gelatinous or skin-like coverings, others have hard eggshells. Softer shells are mostly protein. It may be fibrous or quite liquid. Some arthropod eggs do not actually have shells, rather, their outer covering is actually the outermost extraembryonic membrane, the chorion, which serves to protect inner layers. The chorion itself can be a complex structure, and it may have different layers within it. It may have an outermost layer called an exochorion. Eggs which must survive in dry conditions usually have hard eggshells, made mostly of dehydrated or mineralized proteins with pore systems to allow respiration. Arthropod eggs can have extensive ornamentation on their outer surfaces.

Fish, amphibian and reptile eggs

Fish and amphibians generally lay eggs which are surrounded by the extraembryonic membranes but do not develop a shell, hard or soft, around these membranes. Some fish and amphibian eggs have thick, leathery coats, especially if they must withstand physical force or desiccation.

While many reptiles lay eggs with flexible, calcified, eggshells, there are some that lay hard eggs. Eggs laid by snakes generally have leathery shells which often adhere to one another. Depending on the species, turtles and tortoises lay hard or soft eggs. Several species lay eggs which are nearly indistinguishable from bird eggs. Hatching reptiles, like most birds, have egg-teeth with which they slit or peck an opening into the shell to help them emerge.

Bird eggs

Birds are known for their hard-shelled eggs. The eggshell is approximately 95% calcium carbonate crystals, which are stabilized by a protein matrix. Without the protein, the crystal structure would be too brittle to keep its form. The standard bird eggshell is a porous structure, covered on its outer surface with a cuticle (called the bloom if it is around a chicken egg), which helps the egg retain its water and keep out bacteria. As an egg develops within a bird's reproductive system, the shell is laid down as a liquid mineral layer around the already-formed extraembryonic membranes. This takes place in a part of the bird's oviduct which is called the uterus and serves a very different purpose from the uterus of a mammal. The shell is laid down by shell glands. In an average laying hen, the process of shell formation takes around 20 hours. Pigmentation is added to the shell by papillae lining the oviduct, coloring it any of a variety of colors and patterns depending on species. Since eggs are usually laid blunt end first, that end is subjected to most pressure during its passage and consequently shows the most color. The waxy cuticle is deposited on the shell in the bird's oviduct. The chick has an egg-tooth which it uses to start a hole in the hard eggshell to allow it to hatch.

Mammal eggs

Monotremes, egg-laying mammals, lay soft-shelled eggs similar to those of reptiles. The shell is deposited on the egg in layers within the uterus. The egg can take up fluids and grow in size during this process, and the final, most rigid layer is not added until the egg is full-size. There are three mammals that lay eggs. The platypus, the the short-beaked echidna, and the long-beaked echidna, the echidnas are also known as the spiny anteaters.

The Advantages of Adding Eggshells to Your Garden

Using your empty eggshells in your garden soil is a good way of recycling. The crushed eggshells add healthy and valuable nutrients to your soil. They have a high source of calcium, which is needed for a nutritious cell growth in all types of plants. This is important for fast growing plants because they can go through the source of calcium in the soil quickly. To use the eggshells, first wash them. Then crush them until they are in very small chips. Leaving egg shells whole or in large pieces will take much longer for them to breakdown and be of value to the plants in your garden.

How to Clear Your Drain with Eggshells

With crushed eggshells, you can clear your drain and pipes. To do this follow these three steps. 1) Crush the eggshell as well as you can. One way to crush the shells is to place them in a plastic bag and then pound them with a knife handle or spoon. 2) Place the shells into the drain basket of your sink or tub. 3) Allow the shells to stay in place as the normal use of the sink or tub will eventually send the shells into your pipes.

See also

• Eggshell skull rule, in tort law
• walk on eggshells, an idiom in the English language
• Kilner, R. M. (2006). "The evolution of egg colour and patterning in birds". Biological Reviews. 81: 383–406. doi:10.1017/S1464793106007044.
• Hear the song 'Eggshell Heart' on the Low Slung Podcast