Egg (biology) facts for kids

For the female reproductive cell, see Ovum.

The size of bird eggs changes with the size of the adult bird. An ostrich egg (right) can weigh 1.5 kg. Other eggs shown are from a chicken (lower left) and quail (top left).

An egg is formed after a female cell, called an ovum, is fertilized. Think of an egg as a special container for a tiny new life, called a zygote. It keeps the zygote safe and provides food for the growing embryo inside.

The animal embryo grows inside the egg until it is ready to live on its own. Then, the egg hatches. Many animals, like most vertebrates, arthropods (insects, spiders), and molluscs (snails, clams), lay their eggs outside the mother's body. These eggs always have some kind of protective covering, like a shell.

Reptiles, birds, and monotremes (like platypuses) lay special eggs called cleidoic eggs. Bird eggs are a great example. These eggs have a good supply of food and water. Their outer shell has tiny holes that let gases in and out. This means oxygen can get in for the baby to breathe, and carbon dioxide can leave.

Other animals, like fish, amphibians, insects, and arachnids (spiders), lay simpler eggs. They often lay many more eggs, but these eggs have less protection and food inside.

Largest Animal Eggs

The ostrich lays the biggest eggs of any animal alive today. An ostrich egg can weigh as much as 1.5 kilograms! That's like a small bag of sugar. The shell of an ostrich egg is so strong it can even hold the weight of a grown human.

Many eggs, especially bird eggs, are eaten as food by people. The shell of an egg is made of a hard, chalky material called calcium carbonate.

A baby tortoise hatches from a reptile egg.
Bird eggs with a chick hatching.
A frog with its eggs, called frogspawn.
An ostrich egg.
Eggs in a carton, ready for food.
Butterfly eggs on a plant stalk.
A map butterfly laying strings of eggs.

Eggs in Different Animal Groups

Different animal groups have different types of eggs and ways of developing.

Fish and Amphibian Eggs

Salmon eggs at different stages. You can see the tiny cells growing on the yolk and even the eyes of the baby fish.

A diagram of a fish egg: A. vitelline membrane B. chorion C. yolk D. oil globule E. perivitelline space F. embryo

A baby salmon hatching. The tiny fish has grown around the remains of its yolk sac.

Most fish lay eggs that are fertilized outside their body. The female lays undeveloped eggs, and then the male fertilizes them. Fish often lay a huge number of eggs at once. For example, a female cod fish can lay 4 to 6 million eggs! These eggs are usually left to develop on their own, without parental care.

When baby fish, called larvae, hatch, they often have a yolk sac attached. This yolk sac gives them food for a few days while they learn to swim and find their own food. Once the yolk is gone, they must learn to hunt or they won't survive.

Some fish, like rays and most sharks, keep their eggs inside their body. The eggs are fertilized and develop there, but the babies still get all their food from the egg's yolk. The mother then gives birth to young that are already quite developed. In some rare cases, the strongest baby fish might even eat its smaller siblings inside the mother!

A few fish, like hammerhead sharks, are truly viviparous. This means the eggs are fertilized and develop inside the mother, and she also provides direct food to the babies, not just from the yolk.

Fish and amphibian eggs are usually jelly-like. They don't have a hard shell and would dry out in the air. That's why most fish lay their eggs in water. Even amphibians that breathe air, like frogs, lay their eggs in water or in protective foam.

Bird Eggs and Their Life Cycle

Female birds lay eggs, which are then incubated (kept warm) for a certain time. This time varies depending on the bird species. One young bird hatches from each egg. The number of eggs a bird lays at one time, called a clutch, can range from just one (like in condors) to about 17 (like the grey partridge). Some birds, like hens, can lay eggs even if they haven't been fertilized.

Egg Colors

Guillemot eggs, showing their varied colors.

The natural color of most animal eggs is white, because their shells are made of calcium carbonate. But some birds, especially passerines (songbirds), lay colored eggs. Different pigments (natural colors) give eggs their blue, green, red, or brown colors, or create spots.

Birds that don't sing usually have white eggs. However, ground-nesting birds, like shorebirds, often have camouflaged eggs to blend in with their surroundings and hide from predators. Some parasitic birds, like cuckoos, lay eggs that look like the eggs of the bird whose nest they are using.

Scientists think that the spots on some bird eggs might help make the shell stronger. If there isn't enough calcium in the soil for the bird to get, the eggshell might be thin. The spots can help make up for this and strengthen the shell. Later eggs in a clutch often have more spots because the mother's calcium supply starts to run low.

The color of a bird's eggs is also passed down through its genes, usually from the mother. This suggests that the gene for egg color is on the W chromosome, which determines if a bird is female.

In birds like the common guillemot, which nest in big groups on cliffs, each female's eggs have very unique markings. This helps them find their own eggs among many others on the crowded ledges.

Egg Shells

Bird eggshells come in many different textures. For example:

cormorant eggs feel rough and chalky.
tinamou eggs are very shiny.
duck eggs are oily and waterproof.
cassowary eggs have deep pits.

Bird eggshells have tiny pores, which are like tiny holes. These pores allow the baby bird inside to breathe. A domestic hen's egg has about 7,000 of these tiny pores!

Egg Shape

Most bird eggs are oval-shaped, with one end round and the other more pointed. This shape happens as the egg moves through the mother's oviduct. Muscles push the egg forward, and because the egg's wall is still soft, the pointed end forms at the back.

Birds that fly very well often have long, pointy eggs because their bodies are streamlined for flight, which also makes their oviduct narrower. Birds that nest on cliffs often have very cone-shaped eggs. This is helpful because if they roll, they tend to roll in a tight circle instead of falling off the cliff. Birds that nest in holes often have eggs that are almost perfectly round.

Egg Predation

Many animals like to eat eggs. For example, raccoons, skunks, mink, otters, gulls, crows, and foxes all eat the eggs of the black oystercatcher. Snakes from the Dasypeltis and Elachistodon groups are special because they only eat eggs.

Brood parasitism is when one bird species lays its eggs in another bird's nest. Sometimes, the parasitic bird or its chick will remove or eat the host bird's eggs. Examples of brood parasites include cowbirds and many Old World cuckoos.

Various Egg Examples

Eurasian oystercatcher eggs camouflaged in their nest.
Eggs of ostrich, emu, kiwi and chicken.
A finch egg next to an American dime coin.
Eggs of duck, goose, guineafowl and chicken.
Eggs of ostrich, cassowary, chicken, flamingo, pigeon and blackbird.
An egg from an emu.

Amniote Eggs and Embryos

Turtle eggs in a nest dug by a female common snapping turtle (Chelydra serpentina).

Amniotes are vertebrates that breathe air, like amphibians. But they have more complex eggs or embryos, which include a special protective layer called an amniotic membrane. Amniotes include reptiles (like dinosaurs and birds) and mammals.

Reptile eggs are often rubbery and are always white when first laid. They can survive out of water. For many reptiles, the temperature around the egg determines if the baby will be male or female, with cooler temperatures often leading to males. Not all reptiles lay eggs; some give live birth.

Dinosaurs laid eggs, and some of these have turned into fossils over millions of years.

Among mammals, early types of mammals laid eggs, just like today's platypuses and echidnas (spiny anteaters). These are called monotremes and live in Australia. Marsupials (like kangaroos) and placental mammals (like humans) do not lay eggs. However, their unborn babies still have the complex tissues that make them amniotes.

Mammalian Eggs

The eggs of egg-laying mammals (the platypus and echidnas) are large and full of yolk, much like reptile eggs. Marsupial eggs also have a lot of yolk but are quite small. They develop inside the female's body but don't form a placenta. The young are born very early and are still quite undeveloped.

In placental mammals, the egg itself has no yolk. Instead, the baby develops an umbilical cord from structures that would form the yolk sac in reptiles. The baby gets all its food from the mother through this cord while it grows inside her uterus.

Invertebrate Eggs

Nudibranch Orange-peel doris Acanthodoris lutea laying eggs in a tide pool.

Eggs are very common among invertebrates, which are animals without backbones. This includes insects, spiders, mollusks (like snails and clams), and crustaceans (like crabs and shrimp).

How Eggs Evolved and Are Structured

All living things that reproduce sexually, including plants and animals, create special cells called gametes. The male gamete is a sperm cell, which can usually move. The female gamete is an ovum, which is usually larger and stays in one place. When a sperm and ovum combine, they form a zygote.

In multicellular organisms (animals with many cells), the zygote then divides and grows into an embryo. For most animals, the embryo is the first stage of life and stays in one place. After a while, it hatches and becomes a moving creature. The zygote, the ovum itself, or the container holding the developing embryo can all be called an egg.

Scientists believe that eggs developed later in evolution. They were important for making sure that all the cells in a new multicellular organism were genetically the same.

Scientific Ways to Classify Eggs

Scientists often group animals based on how much the baby develops before it leaves the parent's body. They also classify eggs by how much yolk they have to feed the embryo.

Egg Size and Yolk Content

Vertebrate eggs are classified by how much yolk they contain:

Microlecithal eggs are small with very little yolk.
Mesolecithal eggs are medium-sized with some yolk.
Macrolecithal eggs are large with a lot of yolk.

This system is mainly for chordates (animals with a backbone or similar structure), but the idea applies to all animals.

Microlecithal Eggs

Microlecithal eggs from the roundworm Toxocara.

Microlecithal eggs from the flatworm Paragonimus westermani.

Small eggs with little yolk are called microlecithal. The yolk is spread out evenly. When the egg cell divides, it splits the egg into cells that are all about the same size. In simple animals like sponges and cnidarians (like jellyfish), these dividing eggs quickly grow into a simple larva that has tiny hairs called cilia.

Microlecithal eggs don't need much yolk. They are found in animals like flatworms, roundworms, annelids (worms), bivalves (clams), echinoderms (starfish), and most marine arthropods. These small eggs can be produced in very large numbers. Animals that have many eggs die, like bivalves, often lay microlecithal eggs.

In placental mammals, like humans, the egg is tiny and gets all its food from the mother throughout its development.

Mesolecithal Eggs

Frogspawn, or frog eggs, are mesolecithal.

Mesolecithal eggs have more yolk than microlecithal eggs. The yolk is mostly in one part of the egg (the vegetal pole), while the cell's center and most of the cell material are in the other part (the animal pole). The cell division happens mostly in the animal pole, where there's more cell material.

The larger yolk in mesolecithal eggs allows the baby to develop for a longer time. Simpler animals can fully develop and hatch looking like miniature adults. This happens with hagfish and some snails. Animals with smaller eggs or more complex bodies will still have a distinct larval stage, but the larva will look somewhat similar to the adult. This is seen in lampreys and salamanders.

Macrolecithal Eggs

A baby tortoise hatching "fully developed" from its macrolecithal egg.

Eggs with a very large yolk are called macrolecithal. These eggs are usually few in number, but the babies have enough food to fully develop inside the egg in most cases. Macrolecithal eggs are found in Cephalopods (like squids) and vertebrates.

Macrolecithal eggs develop differently because of the huge yolk. The cell division can't split the yolk mass. Instead, the baby grows as a flat shape on top of the yolk and only surrounds it later. A part of the yolk often remains as a visible yolk sac when the baby hatches. This type of development is common in bony fish, even if their eggs are small.

In land animals with macrolecithal eggs, the large size means they need special parts to help with breathing and storing waste. This is so the baby doesn't suffocate or get poisoned inside the egg. These are called amniote eggs.

Besides bony fish and cephalopods, macrolecithal eggs are found in cartilaginous fish (sharks, rays), reptiles, birds, and monotreme mammals. The eggs of coelacanths, a type of fish, can be 9 cm wide! Their young fully develop inside the mother's uterus, living off the large yolk.

How Animals Reproduce (Egg-laying vs. Live Birth)

Animals are often grouped by how they reproduce: by laying eggs (oviparous) or by giving live birth (viviparous).

Here's a more detailed look:

Ovuliparity: The female lays unfertilized eggs (ova), which are then fertilized outside her body. This is common for bony fish, frogs, and many aquatic animals.
Oviparity: Fertilization happens inside the female, so the eggs she lays already contain a developing baby. These eggs often have important outer layers, like the shell of a chicken egg. Birds, reptiles, some sharks, and most insects are oviparous. Land animals often lay eggs with coverings that prevent them from drying out.
Ovo-viviparity: The fertilized eggs stay inside the mother's body, but the baby still gets all its food from the egg's yolk, not directly from the mother. Most live-bearing fish, amphibians, or reptiles are actually ovoviviparous. Examples include some sharks and certain snakes.
Histotrophic viviparity: Babies develop inside the mother's body and get food by eating other eggs, zygotes, or even their siblings inside the mother. This happens in some sharks and salamanders.
Hemotrophic viviparity: Babies get nutrients directly from the mother's blood, usually through a special organ called a placenta. This is how most mammals reproduce. Similar structures are found in some sharks and lizards.

Human Use of Eggs

Eggs as Food

People have been eating eggs from many different animals for thousands of years. Popular choices include chicken eggs, duck eggs, fish eggs (like roe and caviar). By far, the most commonly eaten egg is the chicken egg, which is usually unfertilized.

Eggs and Kosher Rules

In Kashrut, which are Jewish dietary laws, certain foods are allowed. Kosher meat and milk cannot be mixed. Eggs are considered pareve, meaning they are neither meat nor dairy. This means eggs can be mixed with either milk or kosher meat. For example, Mayonnaise, which contains egg, is usually labeled "pareve."

Vaccine Production

Many vaccines that protect us from infectious diseases are made using fertile chicken eggs. This method was discovered in 1931 by Alice Miles Woodruff and Ernest William Goodpasture. They found that certain bacteria and viruses could grow inside chicken embryos. This discovery led to the creation of vaccines for diseases like influenza, chicken pox, smallpox, yellow fever, and typhus.

Eggs in Culture

Decorating hard-boiled eggs is a popular Easter tradition in some parts of the world. People often dye or paint them. Adults sometimes hide these eggs for children to find in an "Easter egg hunt." A similar tradition of egg painting happens in Persia during the New Year (Norouz). Each family member decorates an egg and places it in a bowl.

In Barcelona, Spain, there's a tradition called the "dancing egg" during the Corpus Christi festival. An empty egg is placed on a fountain's water jet, and it spins without falling off.

Even though eggs are food, they are sometimes thrown at houses, cars, or people. This act, known as "egging," is a minor form of vandalism and can damage property (egg whites can harm car paint) or even cause eye injuries. On Halloween, for example, people might throw eggs if they don't get candy. Eggs are also sometimes thrown during protests because they are cheap and messy but not deadly.

Collecting Eggs

In the past, people sometimes collected eggs as interesting objects or for museums. However, egg collecting has seriously harmed some bird species. Because of this, collecting wild eggs is now illegal in many countries.

Images for kids

Eggs of various birds, a reptile, various cartilaginous fish, a cuttlefish and various butterflies and moths. (Click on image for key)
Six commercial eggs — view from the top against a white background.
Chocolate Easter eggs hidden as part of an egg hunt.
An average whooping crane egg is 102 mm long and weighs 208 g.
Egg of a senegal parrot, a bird that nests in tree holes, on a 1 cm grid.
Egg from a chicken compared to a 1 euro coin, great tit egg and a corn grain.
Bird nest with brown marbling eggs of a robin.
A spherical chicken egg.
Insect eggs, in this case those of the Emperor gum moth, are often laid on the underside of leaves.
Fish eggs, such as these herring eggs are often transparent and fertilized after laying.
Skates and some sharks have a uniquely shaped egg case called a mermaid's purse.
A Testudo hermanni emerging fully developed from a reptilian egg.
A Schistosoma mekongi egg.
Eggs of Huffmanela hamo, a nematode parasite in a fish.
Eggs of various parasites (mainly nematodes) from wild primates.