Egg

An egg is a tiny home.

It holds a baby animal inside. The baby grows until it can live on its own. Then the baby breaks out.

Some eggs have hard shells. Bird eggs have shells that let them breathe. Other eggs are soft and jellylike. Fish and frogs lay these in water.

Eggs can be many sizes. An ostrich egg is very large. A tiny hummingbird egg is very small. Many animals lay eggs to grow their young.

An egg is a special vessel for life.

An animal grows an egg to carry a zygote. A zygote is a single fertilized cell. Inside the egg, an embryo grows. It stays warm until it becomes a fetus. A fetus is a baby that can live on its own. Then, the animal hatches by breaking the shell. Some babies even have a tiny egg tooth to help them crack out.

Eggs come in many forms. Many fish and frogs lay jellylike eggs in water.

These eggs do not have shells. If they were in the air, they would dry out. Reptiles and birds lay eggs on land.

These eggs have protective shells. Bird shells are made of calcium carbonate. They have tiny pores to let the baby breathe.

These pores let in oxygen and let out carbon dioxide.

Eggs also vary in size. The ostrich egg is the largest bird egg. The bee hummingbird has the smallest bird egg. It weighs only half a gram.

Some mammals, like the platypus, even lay eggs!

An egg is a special vessel grown by an animal. It carries a zygote, which is a fertilized egg cell. Inside this vessel, an embryo grows and stays warm. This process continues until the embryo becomes a fetus. A fetus is a baby that can survive on its own. When it is ready, the animal hatches by breaking through the shell.

Some babies even use a tiny egg tooth to crack the shell. This amazing way of life allows many different animals to grow safely.

How an egg works depends on the animal. Many fish and amphibians lay jellylike eggs in the water.

These eggs do not have hard shells, so they would dry out in the air. Some fish use a strategy called oviparity. This is when a female lays undeveloped eggs that a male fertilizes outside her body. Large fish can lay over 100 million eggs at once. When the larvae hatch, they often carry a yolk sac for food.

This yolk helps them learn to swim before they must hunt.

Other animals, called amniotes, lay more complex eggs. This group includes reptiles, birds, and some mammals.

These eggs have a special membrane to help the embryo grow. Reptile eggs can be leathery, like those from snakes. Turtles lay eggs with a hard, calcareous shell. Most reptiles lay their eggs on land in warm, moist places. For some turtles and lizards, the temperature of the nest decides if the baby will be male or female.

Bird eggs are very special because they have hard shells. These shells are made of calcium carbonate. The shell protects the baby from damage and germs. It also has tiny pores to let the embryo breathe.

These pores let oxygen in and carbon dioxide out. A domestic hen's egg has about 7,000 of these tiny holes. Bird eggs come in many shapes and colors. Some are round, while others are long or pointy. Colors come from pigments like biliverdin, which makes eggs green or blue.

Eggs are found all over our world and in our history. The largest recorded egg was from a whale shark. Among birds, the ostrich egg is the largest living kind.

The bee hummingbird has the smallest bird egg, weighing only half a gram. Even some mammals, like the platypus, lay eggs. For humans, eggs are a popular food found on many menus.

They are also important symbols of life and rebirth in many stories. We even use chicken eggs to help make vaccines for diseases.

An egg is an organic vessel grown by an animal to carry a zygote. A zygote is a fertilized egg cell. Inside this protective vessel, an embryo undergoes incubation. This process continues until the embryo develops into a fetus. A fetus is a young animal that can eventually survive on its own. Once development is complete, the animal hatches by breaking out of its shell or covering.

While eggs are common in many animal groups, other reproductive structures exist. In different biological kingdoms, these are called spores, seeds, or egg cells.

The mechanism of development varies greatly depending on the species. In many fish, a strategy called oviparity is used. In oviparity, the female lays undeveloped eggs that are fertilized externally by a male. Many large fish can produce over 100 million eggs in a single spawning. When these larvae hatch, they often possess a yolk sac. This sac provides nourishment for a few days while the larvae learn to swim. Once the yolk is consumed, the larvae reach a critical point where they must hunt or die.

Some sharks and rays use ovoviviparity. In this process, eggs are fertilized and develop internally. The larvae consume the egg's yolk without direct nourishment from the mother before being born.

Amniotes are a specific group of air-breathing vertebrates. This group includes reptiles, birds, and mammals. Amniotes produce complex eggs that include an amniotic membrane. This membrane helps protect the developing embryo. Reptile eggs can be leathery, such as those from snakes, or they can have a calcareous shell, like those from turtles.

These shells allow the eggs to survive in the air. Some reptile eggs can even absorb water from the environment to swell as the fetus grows.

In some turtles and lizards, the temperature of the surroundings determines the sex of the embryo.

Bird eggs are highly specialized for life on land. They possess a hard shell made of calcium carbonate and a 5% organic matrix. This shell prevents desiccation, which is the process of drying out. It also protects against mechanical damage and microbes. The shell is not solid, however, as it contains tiny pores. These pores allow for gas exchange, letting oxygen in and carbon dioxide out.

A domestic hen's egg contains approximately 7,000 of these pores. The distribution of these pores often relates to the length of the incubation period.

The physical characteristics of bird eggs, such as shape and color, serve specific purposes. Egg shapes can be ovoid, spherical, or even highly conical. For example, cliff-nesting birds often lay conical eggs to prevent them from rolling off edges. Coloration is also diverse. While many eggs are white, others use pigments like biliverdin for blue or green tones. Protoporphyrin IX can produce red or brown colors and spotting.

In some species, these markings might even help reduce the brittleness of the shell. The color is secreted by the oviduct shell gland during formation.

Eggs vary immensely in size across the animal kingdom. The largest recorded egg came from a whale shark. Among living birds, the ostrich egg is the largest, reaching up to 18 centimeters in length. In contrast, the bee hummingbird produces the smallest known bird egg. This tiny egg weighs only about half a gram.

Some invertebrates, such as insects or fish, can produce eggs that are even smaller than those of the bee hummingbird. These extremes show how eggs are adapted to different life strategies.

Beyond biology, eggs hold significant meaning for human society. They are a popular food item used in cuisines worldwide. In science, chicken eggs are even used in the production of vaccines for infectious diseases.

Eggs also serve as powerful symbols in folklore and mythology. They often represent life, healing, and rebirth. Humans have also used eggs for decoration and collecting throughout history. Even though some forms of egg collecting are now banned, the egg remains a central part of many cultural traditions.