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An egg is an organic vessel grown by an animal to carry a possibly
Reptile eggs, bird eggs, and monotreme eggs are laid out of water and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches, i.e., breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering.
The largest recorded egg is from a
Eggs of different animal groups
Several major groups of animals typically have readily distinguishable eggs.
|Class||Types of eggs||Development|
|Jawless fish||Mesolecithal eggs, especially large in hagfish|
|Cartilaginous fish||Macrolecithal eggs with egg capsule||Direct development, viviparity in some species|
|Bony fish||Macrolecithal eggs, small to medium size, large eggs in the coelacanth||Larval stage, ovovivipary in some species.|
|Amphibians||Medium-sized mesolecithal eggs in all species.||Tadpole stage, direct development in some species.|
|Large macrolecithal eggs, develop independent of water.||Direct development, some ovoviviparious|
|Large to very large macrolecithal eggs in all species, develop independent of water.||The young more or less fully developed, no distinct larval stage.|
|Macrolecithal eggs in
|Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals.|
Fish and amphibian eggs
The most common reproductive strategy for fish is known as oviparity, in which the female lays undeveloped eggs that are externally fertilized by a male. Typically large numbers of eggs are laid at one time (an adult female cod can produce 4–6 million eggs in one spawning) and the eggs are then left to develop without parental care. When the larvae hatch from the egg, they often carry the remains of the yolk in a yolk sac which continues to nourish the larvae for a few days as they learn how to swim. Once the yolk is consumed, there is a critical point after which they must learn how to hunt and feed or they will die.
A few fish, notably the
In certain scenarios, some fish such as the
The eggs of fish and
Bird eggs are laid by females and
The default color of vertebrate eggs is the white of the
Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay colored eggs, even if there is no need of cryptic colors.
However some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid-state lubricant. If there is insufficient calcium available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil.
For the same reason, later eggs in a clutch are more spotted than early ones as the female's store of calcium is depleted.
The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting that the
It used to be thought that color was applied to the shell immediately before laying, but subsequent research shows that coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral.
In species such as the
Bird eggshells are diverse. For example:
- cormorant eggs are rough and chalky
- tinamou eggs are shiny
- duck eggs are oily and waterproof
- cassowary eggs are heavily pitted
Some bird eggshells have a coating of
Most bird eggs have an
Many animals feed on eggs. For example, principal predators of the
An average whooping crane egg is 102 mm (4.0 in) long and weighs 208 g (7.3 oz)
Eurasian oystercatcher eggs camouflaged in the nest
Egg of a senegal parrot, a bird that nests in tree holes, on a 1 cm (0.39 in) grid
Eggs ofpigeon and blackbird
Egg of an emu
Egg from a chicken compared to a 1 euro coin, great tit egg and a corn grain
A spherical chicken egg
Amniote eggs and embryos
Reptile eggs are often rubbery and are always initially white. They are able to survive in the air. Often the sex of the developing embryo is determined by the temperature of the surroundings, with cooler temperatures favouring males. Not all reptiles lay eggs; some are
Dinosaurs laid eggs, some of which have been preserved as petrified fossils.
Among mammals, early extinct species laid eggs, as do
The eggs of the egg-laying mammals (the platypus and the echidnas) are macrolecithal eggs very much like those of reptiles. The eggs of marsupials are likewise macrolecithal, but rather small, and develop inside the body of the female, but do not form a placenta. The young are born at a very early stage, and can be classified as a "larva" in the biological sense.
Eggs are common among
Evolution and structure
All sexually reproducing life, including both plants and animals, produces
A recent proposal suggests that the phylotypic animal body plans originated in cell aggregates before the existence of an egg stage of development. Eggs, in this view, were later evolutionary innovations, selected for their role in ensuring genetic uniformity among the cells of incipient multicellular organisms.
The cycle of the egg's formation is started by the
Scientists often classify animal reproduction according to the degree of development that occurs before the new individuals are expelled from the adult body, and by the yolk which the egg provides to nourish the embryo.
Egg size and yolk
Small eggs with little yolk are called microlecithal. The yolk is evenly distributed, so the cleavage of the egg cell cuts through and divides the egg into cells of fairly similar sizes. In
Microlecithal eggs require minimal yolk mass. Such eggs are found in
In placental mammals, where the embryo is nourished by the mother throughout the whole fetal period, the egg is reduced in size to essentially a naked egg cell.
Mesolecithal eggs have comparatively more yolk than the microlecithal eggs. The yolk is concentrated in one part of the egg (the vegetal pole), with the cell nucleus and most of the cytoplasm in the other (the animal pole). The cell cleavage is uneven, and mainly concentrated in the cytoplasma-rich animal pole.
The larger yolk content of the mesolecithal eggs allows for a longer fetal development. Comparatively anatomically simple animals will be able to go through the full development and leave the egg in a form reminiscent of the adult animal. This is the situation found in hagfish and some snails. Animals with smaller size eggs or more advanced anatomy will still have a distinct larval stage, though the larva will be basically similar to the adult animal, as in lampreys, coelacanth and the salamanders.
Eggs with a large yolk are called macrolecithal. The eggs are usually few in number, and the embryos have enough food to go through full fetal development in most groups. Macrolecithal eggs are only found in selected representatives of two groups: Cephalopods and vertebrates.
Macrolecithal eggs go through a different type of development than other eggs. Due to the large size of the yolk, the cell division can not split up the yolk mass. The fetus instead develops as a plate-like structure on top of the yolk mass, and only envelopes it at a later stage.
In addition to bony fish and cephalopods, macrolecithal eggs are found in cartilaginous fish, reptiles, birds and monotreme mammals. The eggs of the coelacanths can reach a size of 9 cm (3.5 in) in diameter, and the young go through full development while in the uterus, living on the copious yolk.
Animals are commonly classified by their manner of reproduction, at the most general level distinguishing egg-laying (Latin. oviparous) from live-bearing (Latin. viviparous).
These classifications are divided into more detail according to the development that occurs before the offspring are expelled from the adult's body. Traditionally:
- Ovuliparity means the female anurans, echinoderms, bivalves and cnidarians. Most aquatic organisms are ovuliparous. The term is derived from the diminutive meaning "little egg".
- Oviparity is where fertilisation occurs internally and so the eggs laid by the female are zygotes (or newly developing embryos), often with important outer tissues added (for example, in a chicken egg, no part outside of the yolk originates with the zygote). Oviparity is typical of birds, reptiles, some cartilaginous fish and most arthropods. Terrestrial organisms are typically oviparous, with egg-casings that resist evaporation of moisture.
- Ovo-viviparity is where the zygote is retained in the adult's body but there are no trophic (feeding) interactions. That is, the embryo still obtains all of its nutrients from inside the egg. Most live-bearing fish, amphibians or reptiles are actually ovoviviparous. Examples include the reptile Anguis fragilis, the sea horse (where zygotes are retained in the male's ventral "marsupium"), and the frogs Rhinoderma darwinii (where the eggs develop in the vocal sac) and Rheobatrachus (where the eggs develop in the stomach).
- Histotrophic viviparity means embryos develop in the female's Marsupials excrete a "uterine milk" supplementing the nourishment from the yolk sac.
- Hemotrophic viviparity is where nutrients are provided from the female's blood through a designated organ. This most commonly occurs through a placenta, found in most mammals. Similar structures are found in some sharks and in the lizard Pseudomoia pagenstecheri. In some hylid frogs, the embryo is fed by the mother through specialized gills.
The term hemotropic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.
Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia. Popular choices for egg consumption are chicken, duck, roe, and caviar, but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.
Eggs and Kashrut
According to the
Many vaccines for infectious diseases are produced in fertile chicken eggs. The basis of this technology was the discovery in 1931 by
Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths.
Although a food item, raw eggs are sometimes thrown at houses, cars, or people. This act, known commonly as "egging" in the various English-speaking countries, is a minor form of vandalism and, therefore, usually a criminal offense and is capable of damaging property (egg whites can degrade certain types of vehicle paint) as well as potentially causing serious eye injury. On Halloween, for example, trick or treaters have been known to throw eggs (and sometimes flour) at property or people from whom they received nothing. Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken.
Collecting eggs of wild birds is now banned by many jurisdictions, as the practice can threaten rare species. In the United Kingdom, the practice is prohibited by the Protection of Birds Act 1954 and Wildlife and Countryside Act 1981. On the other hand, ongoing underground trading is becoming a serious issue.
Since the protection of wild bird eggs was regulated, early collections have come to the museums as curiosities. For example, the Australian Museum hosts a collection of about 20,000 registered clutches of eggs, and the collection in Western Australia Museum has been archived in a gallery. Scientists regard egg collections as a good natural-history data, as the details recorded in the collectors' notes have helped them to understand birds' nesting behaviors.
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.
- mermaid's purse.
ATestudo hermanniemerging fully developed from a reptilian egg.
A Schistosoma mekongi egg.
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