Egg

Aneggis an organic vessel grown by an animal to carry a possiblyfertilizedegg cell(azygote) and toincubatefrom it anembryowithin the egg until the embryo has become an animalfetusthat can survive on its own, at which point the animal hatches.
Mostarthropods,vertebrates(excludinglive-bearing mammals), andmolluskslay eggs, although some, such asscorpions, do not.
Reptileeggs,bird eggs, andmonotremeeggs are laid out of water and are surrounded by a protectiveshell, 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 temporaryegg tooththey use to crack, pip, or break the eggshell or covering.
The largest recorded egg is from awhale sharkand was 30 cm × 14 cm × 9 cm (11.8 in × 5.5 in × 3.5 in) in size.Whale shark eggs typically hatch within the mother. At 1.5 kg (3.3 lb) and up to 17.8 cm × 14 cm (7.0 in × 5.5 in), theostrichegg is the largest egg of any living bird,though the extinctelephant birdand somenon-avian dinosaurslaid larger eggs. Thebee hummingbirdproduces the smallest known bird egg, which measures between 6.35–11.4 millimetres (0.250–0.449 in) long and weighs half of a gram (around 0.02 oz).Some eggs laid by reptiles and most fish, amphibians, insects, and otherinvertebratescan be even smaller.
Reproductive structures similar to the egg in otherkingdomsare termed "spores", or inspermatophytes"seeds", or ingametophytes"egg cells".
Several major groups of animals typically have readily distinguishable eggs.
The most common reproductive strategy forfishis known asoviparity, 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 femalecodcan 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 theraysand mostsharksuseovoviviparityin which the eggs are fertilized and develop internally. However, the larvae still grow inside the egg consuming the egg's yolk and without any direct nourishment from the mother. The mother then gives birth to relatively mature young. In certain instances, the physically most developed offspring will devour its smaller siblings for further nutrition while still within the mother's body. This is known asintrauterine cannibalism.
In certain scenarios, some fish such as thehammerhead sharkandreef sharkareviviparous, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment.
The eggs of fish andamphibiansare jellylike. Cartilaginous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development. Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog,Chiromantis xerampelina.
Bird eggs are laid by females andincubatedfor a time that varies according to the species; a single young hatches from each egg. Averageclutch sizesrange from one (as incondors) to about 17 (thegrey partridge). Some birds lay eggs even when not fertilized (e.g.hens); it is not uncommon for pet owners to find their lone bird nesting on a clutch of unfertilized eggs, which are sometimes called wind-eggs.
The default colour of vertebrate eggs is the white of thecalcium carbonatefrom which the shells are made, but some birds, mainlypasserines, produce coloured eggs. The colour comes frompigmentsdeposited on top of the calcium carbonate base;biliverdinand its zincchelate, andbilirubin, give a green or blue ground colour, whileprotoporphyrin IXproduces reds and browns as a ground colour or as spotting.
Non-passerines typically have white eggs, except in some ground-nesting groups such as theCharadriiformes,sandgrouseandnightjars, where camouflage is necessary, and someparasiticcuckooswhich have to match the passerine host's egg. Most passerines, in contrast, lay coloured 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 thegeneresponsible for pigmentation is on the sex-determiningW chromosome(female birds are WZ, males ZZ).
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 thecommon guillemot, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed.
Yolksof birds' eggs are yellow fromcarotenoids, it is affected by their living conditions and diet.
Bird eggshells are diverse. For example:
Tiny pores in bird eggshells allow the embryo to breathe. Thedomestic hen's egg has around 7000 pores.
Some bird eggshells have a coating ofvateritespherules, which is a rare polymorph of calcium carbonate. InGreater AniCrotophaga majorthis vaterite coating is thought to act as a shock absorber, protecting the calcite shell from fracture during incubation, such as colliding with other eggs in the nest.
Most bird eggs have anoval shape, with one end rounded and the other more pointed. This shape results from the egg being forced through theoviduct. Muscles contract the oviduct behind the egg, pushing it forward. The egg's wall is still shapeable, and the pointed end develops at the back.One hypothesis is that long, pointy eggs are an incidental consequence of having a streamlined body typical of birds with strong flying abilities; flight narrows the oviduct, which changes the type of egg a bird can lay.
Cliff-nesting birds often have highlyconicaleggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearlysphericaleggs.
Many animals feed on eggs. For example, principal predators of theblack oystercatcher's eggs includeraccoons,skunks,mink, river and seaotters,gulls,crowsandfoxes. Thestoat(Mustela erminea) andlong-tailed weasel(M. frenata) steal ducks' eggs.Snakesof the generaDasypeltisandElachistodonspecialize in eating eggs.
Brood parasitismoccurs in birds when one species lays its eggs in the nest of another. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include thecowbirdsand many Old Worldcuckoos.
Like amphibians,amniotesare air-breathingvertebrates, but they have complex eggs orembryos, including anamniotic membrane. Amniotes includereptiles(including dinosaurs and their descendants, birds) and mammals.
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 areviviparous("live birth").
Dinosaurs laid eggs, some of which have been preserved as petrified fossils.
Among mammals, early extinct species laid eggs, as doplatypusesandechidnas(spiny anteaters). Platypuses and two genera of echidna are Australianmonotremes.Marsupialandplacentalmammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.
The eggs of the egg-laying mammals (theplatypusand theechidnas) are macrolecithal eggs very much like those ofreptiles. The eggs ofmarsupialsare likewise macrolecithal, but rather small, and develop inside the body of the female, but do not form aplacenta. The young are born at a very early stage, and can be classified as a "larva" in the biological sense.
Inplacental mammals, the egg itself is void of yolk, but develops anumbilical cordfrom structures that in reptiles would form the yolk sac. Receiving nutrients from the mother, the fetus completes the development while inside the uterus.
Eggs are common amonginvertebrates, includinginsects,spiders,mollusks, andcrustaceans.
All sexually reproducing life, including both plants and animals, producesgametes. The male gamete cell,sperm, is usually motile whereas the female gamete cell, theovum, is generally larger andsessile. The male and female gametes combine to produce thezygotecell. Inmulticellularorganisms, the zygote subsequently divides in an organised manner into smaller more specialised cells, so that this new individual develops into anembryo. In most animals, the embryo is the sessile initial stage of the individual life cycle, and is followed by the emergence (that is, the hatching) of a motile stage. The zygote or the ovum itself or the sessile organic vessel containing the developing embryo may be called the egg.
A recent proposal suggests that thephylotypicanimalbody plansoriginated in cell aggregates before the existence of an egg stage ofdevelopment. Eggs, in this view, were later evolutionaryinnovations, 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 thegameteovumbeing released (ovulated) and egg formation being started. The finished egg is thenovipositionedand eventualegg incubationcan start.
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.
Vertebrateeggs can be classified by the relative amount ofyolk. Simple eggs with little yolk are calledmicrolecithal, medium-sized eggs with some yolk are calledmesolecithal, and large eggs with a large concentrated yolk are calledmacrolecithal.This classification of eggs is based on the eggs ofchordates, though the basic principle extends to the wholeanimal kingdom.
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. Inspongesandcnidarians, the dividing eggs develop directly into a simple larva, rather like amorulawithcilia. In cnidarians, this stage is called theplanula, and either develops directly into the adult animals or forms new adult individuals through a process ofbudding.
Microlecithal eggs require minimal yolk mass. Such eggs are found inflatworms,roundworms,annelids,bivalves,echinoderms, thelanceletand in most marinearthropods.In anatomically simple animals, such as cnidarians and flatworms, the fetal development can be quite short, and even microlecithal eggs can undergo direct development. These small eggs can be produced in large numbers. In animals with high egg mortality, microlecithal eggs are the norm, as in bivalves and marine arthropods. However, the latter are more complex anatomically than e.g. flatworms, and the small microlecithal eggs do not allow full development. Instead, the eggs hatch intolarvae, which may be markedly different from the adult animal.
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 (thevegetal pole), with thecell nucleusand most of thecytoplasmin the other (theanimal 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 inhagfishand somesnails.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 inlampreys,coelacanthand thesalamanders.
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:Cephalopodsandvertebrates.
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.A portion of the yolk mass is still present as an external or semi-externalyolk sacat hatching in many groups. This form of fetal development is common inbony fish, even though their eggs can be quite small. Despite their macrolecithal structure, the small size of the eggs does not allow for direct development, and the eggs hatch to a larval stage ("fry"). In terrestrial animals with macrolecithal eggs, the large volume to surface ratio necessitates structures to aid in transport of oxygen and carbon dioxide, and for storage of waste products so that the embryo does not suffocate or get poisoned from its own waste while inside the egg, seeamniote.
In addition to bony fish and cephalopods, macrolecithal eggs are found incartilaginous fish,reptiles,birdsandmonotrememammals.The eggs of thecoelacanthscan reach a size of 9 cm (3.5 in) in diameter, and the young go through full development while in theuterus, 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:
The term hemotrophic 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.
According to theKashrut, that is the set of Jewishdietary laws,kosherfood may be consumed according tohalakha(Jewish law).Eggsare consideredpareve(neither meat nor dairy) despite being an animal product and can be mixed with either milk or kosher meat.
Many vaccines for infectious diseases are produced in fertile chicken eggs. The basis of this technology was the discovery in 1931 byAlice Miles WoodruffandErnest William GoodpastureatVanderbilt Universitythat therickettsiaandvirusesthat cause a variety of diseases will grow in chicken embryos. This enabled the development of vaccines againstinfluenza,chicken pox,smallpox,yellow fever,typhus,Rocky mountain spotted feverand other diseases.
Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths.Egg decorationis a common practice in many cultures worldwide. Christians viewEaster eggsas symbolic of the resurrection of Jesus Christ.A popular Easter tradition in some parts of the world is the decoration of hard-boiled eggs (usually by dyeing, but often by hand-painting or spray-painting). Adults often hide the eggs for children to find, an activity known as anEaster egg hunt. A similar tradition of egg painting exists in areas of the world influenced by the culture of Persia. Before the spring equinox in the Persian New Year tradition (called Norouz), each family member decorates a hard-boiled egg and sets them together in a bowl. The tradition of adancing eggis held during the feast of Corpus Christi in Catalan cities since the 16th century. It consists of an emptied egg, positioned over the water jet from a fountain, which starts turning without falling.
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.
Egg collecting was a popular hobby in some cultures, includingEuropean Australians. Traditionally, the embryo would be removed before a collector stored the egg shell.
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 andWildlife and Countryside Act 1981.However, illegal collection and trading persists.
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.