Egg

An egg is a special container that comes from many animals. Inside this container, a tiny baby animal starts to grow. This tiny baby is called an embryo, and it stays safe inside the egg until it is ready to live on its own. Then, the baby animal breaks out of the egg, which we call hatching.

Many animals lay eggs, like birds, reptiles, and insects. Some birds, such as chickens, lay eggs with hard shells to protect the baby inside. These eggs often need to stay warm so the baby can grow properly.

For people, eggs are a popular food that you can find on menus all over the world. Eggs also have a special meaning in stories and myths, where they often represent new life and fresh starts. People sometimes decorate eggs, making them beautiful and colorful. In the past, some people collected eggs, but this practice is not allowed in many places today. Eggs are also important in making medicines for certain diseases.

Eggs of different animal groups

The largest recorded egg comes from a whale shark, measuring about 30 cm by 14 cm by 9 cm. Whale shark eggs hatch inside the mother. The ostrich egg, at 1.5 kg and up to 17.8 cm by 14 cm, is the largest egg from any living bird, though some extinct birds and dinosaurs laid larger eggs. The smallest bird egg comes from the bee hummingbird, measuring just 6.35 to 11.4 millimetres long and weighing half a gram.

Different animal groups have eggs that look and act in unique ways. Fish and amphibians often lay many small eggs that develop in water. Some fish, like sharks, keep their eggs inside the mother’s body until they hatch. Birds lay eggs with hard shells that protect the developing embryo. Mammals like the platypus are among the few that still lay eggs, while most mammals develop their young inside the mother’s body. Invertebrates, such as insects and spiders, also lay eggs, sometimes caring for their young in special ways.

Fish and amphibian eggs

"Fish egg" redirects here. For fish eggs as food, see Roe.

See also: Ichthyoplankton and Spawn (biology)

Most fish lay eggs that are fertilized outside the body. These eggs often have a yolk that feeds the baby fish until they can swim and find food. Some fish, like sharks, keep their eggs inside until they are ready to hatch. Fish and amphibians eggs are usually jellylike and need to be in water to develop.

Amniote eggs and embryos

Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or embryos, including an amniotic membrane. The shelled egg is what gives amniotes their name. Amniotes include reptiles (including birds) and mammals.

Reptile eggs have different types of shells, from leathery to hard, and are usually laid on land. The sex of baby reptiles can depend on the temperature around the egg. Some reptiles give birth to live young instead of laying eggs. Dinosaurs also laid eggs, some of which have been found as fossils.

Among mammals, only the platypuses and echidnas lay eggs. Most other mammals develop their young inside the mother’s body.

Bird eggs

Main article: Bird egg

Bird eggs are laid by females and usually have one baby inside. Bird eggshells are made of calcium carbonate and help keep the egg safe. The shells can be different colors and shapes depending on the bird. Some birds lay eggs with colors that help hide them, while others lay white eggs.

Shell

Bird eggs have a hard shell made of calcium carbonate with a small amount of organic material. This shell keeps the egg from drying out and protects it from damage and germs. Tiny holes in the shell let the baby bird breathe. The shell can be very thin or quite thick, depending on the bird.

Shape

Bird eggs are usually oval but can be different shapes. Some birds, like those that fly fast, have pointier eggs. Birds that nest on cliffs often have conical eggs that don’t roll away easily.

Colors

Most bird eggs are white, but some birds lay colored eggs. The color comes from special materials added to the shell. Some colors help hide the eggs, while others might help the parents take better care of them.

Predation

Many animals eat bird eggs. Some birds lay their eggs in other birds’ nests, a behavior called brood parasitism. This can make it hard for the host birds to recognize their own eggs.

Mammalian eggs

The eggs of egg-laying mammals like the platypus and echidnas are similar to reptile eggs. Most mammals, however, develop their young inside the mother’s body without laying eggs.

Invertebrate eggs

Eggs are common in invertebrates like insects, spiders, mollusks, and crustaceans. Some of these eggs have more yolk to help the baby develop before hatching, while others hatch quickly into babies that can look for food. A few invertebrates, like the tsetse fly, keep their young inside their body until they are ready to hatch. Some insects can make babies without a father by a process called parthenogenesis.

Evolution and structure

All animals and plants that reproduce need special cells called gametes. The male gamete, called sperm, can move, while the female gamete, called the ovum, is usually larger and stays in one place. When sperm and ovum join, they form a zygote. In animals with more than one cell, this zygote splits into smaller cells and grows into an embryo. Most animals start their lives as a resting embryo before they hatch and move.

Eggs give the embryo food through a part called yolk. The main proteins in yolk help feed the growing embryo. Another part of the egg, called egg white or albumen, holds water for the embryo. The most common protein in egg white is ovalbumin.

Formation

An egg begins when a special cell called an ovum is released and starts the process of egg formation. As it moves through the oviduct, layers like the albumen, shell membranes, and the outer shell are added. Once the egg is complete, it is laid and can then begin egg incubation.

Scientific classifications

Scientists study how animals reproduce by looking at how developed the baby is before it leaves the parent's body and how much food (yolk) the egg provides.

Animal eggs can be grouped by how much yolk they have. Eggs with little yolk are called microlecithal, those with medium yolk are mesolecithal, and eggs with a lot of yolk are macrolecithal. These groups help scientists understand how babies grow inside the egg.

Some animals lay eggs to reproduce, while others give birth to live young. Scientists call animals that lay eggs "oviparous" and those that give birth "viviparous." There are even more detailed ways to describe reproduction, like whether the baby gets food from the egg or from the parent.

Human use

Food

People have eaten eggs from birds, reptiles, amphibians, and fish for thousands of years. The most common egg eaten today comes from chickens, usually ones that are not fertilized.

Eggs and Kashrut

See also: Kashrut § Pareve foods, and Kosher foods § Eggs

According to Kashrut, the Jewish dietary laws, kosher food can be eaten following certain rules. Eggs are considered neutral, meaning they are not meat or dairy, so they can be mixed with either milk or kosher meat.

Vaccine manufacture

Many medicines to protect against sickness are made using special chicken eggs. Scientists found that certain germs can grow inside these eggs, which helps make vaccines for diseases like influenza, chicken pox, smallpox, and yellow fever.

Culture

Eggs appear in stories and traditions around the world, often standing for new life and fresh starts. Many people decorate eggs, especially during holidays like Easter, when Easter eggs remind Christians of new life.

Sometimes, people throw raw eggs as a prank or during protests. This is called "egging" and can damage cars or hurt someone’s eyes, so it is usually against the law.

Collecting

Main article: Oology

Egg collecting used to be a popular hobby in some places, including among European Australians. People would carefully take the baby inside the egg out before keeping the shell.

Now, many places have made it illegal to collect eggs from wild birds because it can harm rare species. For example, in the United Kingdom, laws like the Protection of Birds Act 1954 and the Wildlife and Countryside Act 1981 stop this practice. Even so, some people still collect and trade eggs illegally.

Because of these laws, old egg collections are now kept in museums. Museums like the Australian Museum have many egg collections that help scientists learn about how birds nest and behave.