Anegg is an organic vessel grown by an animal to carry a possiblyfertilizedegg cell – azygote. Within the vessel, anembryo isincubated until it has become an animalfetus that can survive on its own, at which point the animal hatches. Reproductive structures similar to the egg in otherkingdoms are termed "spores", or inspermatophytes "seeds", or ingametophytes "egg cells".
Mostarthropods,vertebrates (excludinglive-bearing mammals), andmollusks lay eggs, although some, such asscorpions, do not.Reptile eggs,bird eggs, andmonotreme eggs 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 tooth they use to crack, pip, or break the eggshell or covering.
For people,eggs are a popular food item and they appear on menus worldwide. Eggs remain an important symbol in folklore and mythology, symbolizing life, healing, and rebirth. They are frequently the subject ofdecoration. Egg collection has been a popular hobby in some cultures, although the practice is now banned. Chicken eggs are used in the production of vaccines forinfectious diseases.
The largest recorded egg is from awhale shark and was 30 cm × 14 cm × 9 cm (11.8 in × 5.5 in × 3.5 in) in size.[1] Whale shark eggs typically hatch within themother. At 1.5 kg (3.3 lb) and up to 17.8 cm × 14 cm (7.0 in × 5.5 in), theostrich egg is the largest egg of any living bird,[2]: 130 though the extinctelephant bird and somenon-avian dinosaurs laid larger eggs. Thebee hummingbird produces 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).[2]: 132 Some eggs laid by reptiles and most fish, amphibians, insects, and otherinvertebrates can be even smaller.
Several major groups of animals typically have readily distinguishable eggs.
Salmon eggs in different stages of development. In some only a few cells grow on top of theyolk, in the lower right the blood vessels surround the yolk and in the upper left the black eyes are visible.
Salmon fry hatching. The larva has grown around the remains of the yolk and the remains of the soft, transparent egg are discarded.
The most common reproductive strategy forfish is known asoviparity,[10] in which the female lays undeveloped eggs that are externally fertilized by a male.[11] Typically large numbers of eggs are laid at one time (large fish are capable of producing over 100 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.[12]
A few fish, notably therays and mostsharks useovoviviparity in 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.[13][14]
In certain scenarios, some fish such as thehammerhead shark andreef shark areviviparous, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment.[15]
Moor frog eggs utilizeglycans to form a hydrophilic jelly coat that protects the egg[16]
The eggs of fish andamphibians (anamniotes) are jellylike.[17] Cartilaginous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development.[18] Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them.[11] These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water,[17] or in protective foam as with the Coast foam-nest treefrog,Chiromantis xerampelina.[19]
Like amphibians,amniotes are air-breathingvertebrates, but they have complex eggs orembryos, including anamniotic membrane.[20] (The shelled egg is the source for the name Amniota.) The formation of this type of egg requires that conception take place internally, and the shell isolates the embryo development from the mother. Amniotes includereptiles (including dinosaurs and their descendants, birds) and mammals.[21]
Reptile eggs are leathery for snakes and the majority of lizards, while turtles have acalcareous shell. These protective shells are able to survive in the air. They will absorb water from the environment, causing them to swell in size while the fetus is developing. Most reptile eggs are deposited on land, usually in a warm, moist environment, then left alone by the parents.[22] Initially, they are always white. For turtles,tuatara, and most lizards, the sex of the developing embryo is determined by the temperature of the surroundings, with the species determining which gender is favored at cool versus warm temperatures.[23] Not all reptiles lay eggs; some areviviparous ("live birth"). This adaptation may have allowed reptiles to inhabit new habitats, especially in colder climates.[24]
Dinosaurs laid eggs, some of which have been preserved as petrified fossils. Soft-shelled dinosaur eggs are less likely to be preserved, so most of the recovered fossilized egg remains come from calcified eggshells.[25]
Among mammals, early extinct species were found to lay eggs, and was probably the ancestral state.[21]Platypuses and two genera ofechidna (spiny anteaters) are Australianmonotremes, the only order of extant egg-laying mammal.[26]Marsupial andplacental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.[21]
Bird eggs
Diagram of a fertilizedchicken egg in its ninth daySix commercial chicken eggs
Bird eggs are laid by females andincubated for a time that varies according to the species;[27] normally a single young hatches from each egg. Twin yolk eggs have been observed in domestic fowl, but this results in low hatchability.[28] One case of twin geese has been observed to hatch from an elongated egg.[29] Averageclutch sizes range from one (as incondors[30]) to about 17–24 (thegrey partridge[31]). It is rare for a bird to lay eggs when not fertilized,[32] known asparthenogenesis. One exception is the domestichen; it is not uncommon for pet owners to find their lone bird nesting on a clutch of unfertilized eggs,[33] which are sometimes called wind-eggs.[34]
Shell
Bird eggs have a hard shell made ofcalcium carbonate with a 5% organic matrix. This resilient external surface preventsdesiccation of the contents, limits mechanical damage, and protects against microbes, all while allowing the exchange of gas with the surrounding atmosphere.[35] They vary in thickness from paper thin up to2.7 mm inostriches, and typically form11%–15% of the egg's weight.[36] Bird eggshells are diverse in appearance and structure.[36] For example:
Tiny pores in bird eggshells allow the embryo to breathe; exchanging oxygen, carbon dioxide, and water with the environment. The pore distribution varies by species, with the pore size being inversely proportional to the incubation period.[38] Thedomestic hen's egg has around 7000 pores.[39]
Some bird eggshells have a coating ofvateritespherules, which is a rare polymorph of calcium carbonate. InGreater AniCrotophaga major this 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.[40]
Bird egg shapes areovoid andaxisymmetrical in form, but vary byellipticity andasymmetry depending on the bird species. Thus, thebrown boobook species has a nearly spherical shell, themaleo egg is highly ellipsoidal, and theleast sandpiper egg is much more conical. The shape is likely formed as the egg moves through the final part of the oviduct, being initially more spherical in form. Ellipticity is introduced by the egg being easier to stretch along the oviduct axis. The eggs of birds that have adapted for high-speed flight often have a more elliptical or asymmetrical form. Thus, 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.[41][42]
Cliff-nesting birds often have highlyconical eggs. 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 nearlyspherical eggs.[43]
The default colour of avian eggs is the white of thecalcium carbonate from which the shells are made, but some birds, mainlypasserines, produce coloured eggs. The colour comes frompigments deposited on top of the calcium carbonate base;biliverdin and its zincchelate, andbilirubin, give a green or blue ground colour, whileprotoporphyrin IX produces reds and browns as a ground colour or as spotting.[44][45][46] Shell colours are secreted by the sameoviduct shell gland that generates the egg shell, and thus can be deposited throughout the shell. When a chalky covering is added, it is the final step in the process.[47]
Non-passerines typically have white eggs,[48] except in some ground-nesting groups such as theCharadriiformes,[48]sandgrouse,[49] andcommon terns,[50] where camouflage is necessary, and someparasiticcuckoos which have to match the passerine host's egg.[51] Most passerines, in contrast, lay coloured eggs, even if there is no need of cryptic colours. However, some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid-state lubricant.[52] 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.[53] Later eggs in a clutch are more spotted than early ones as the female's pigment glands become depleted.[46]
Within thecommon cuckoo lineage, the colour of individual eggs is genetically influenced, and appears to be inherited through the mother only. This suggests that thegene responsible for pigmentation is on the sex-determiningW chromosome (female birds are WZ, males ZZ). However, egg colour in other species is most likely inherited from both parents.[54] For chickens, egg colour appears determined from the hen's genome, diet, and stress factors like disease.[55] WithAmerican robins, there is some evidence that the brightness of the egg colouration may influence male parental care of the nestlings.[56]
Evolutionary factors can drive egg colouration, such aspredation selecting forcryptic colouration, or colourful eggs possibly being used to coerce males into providing additional care during incubation – theblackmail hypothesis.[57] For avian species that play host tobrood parasite eggs, selection pressure drives the host species to evolve distinctive egg colourations so that foreign eggs can be identified and rejected. Likewise, the brood parasite species evolve eggs that better mimic those of the host. The result is an egg colourationevolutionary arms race between the host and parasite.[58] 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.[59]
Yolks of birds' eggs are yellow fromcarotenoids, it is affected by their living conditions and diet.[44]
Brood parasitism occurs in birds when one species lays its eggs in the nest of another. This is an uncommon behavior, with 1% of bird species beingobligate parasites.[58] In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick.[65] Brood parasites include thecowbird,black-headed duck,cuckoo-finch, and three Old Worldcuckoo species.[58]
Mammalian eggs
The eggs of the egg-laying mammals (theplatypus and theechidnas) are macrolecithal eggs very much like those ofreptiles. The eggs ofmarsupials are 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.[66]
Inplacental mammals, there are two types of placenta: theyolk sac and thechorioallantoic. In humans, the initial nutrient source is a yolk sac placenta that is replaced by a chorioallantoic placenta at around four weeks. Around the eighth week, the yolk sac is absorbed into theumbilical cord.[67] Receiving nutrients from the mother, the fetus completes the development while inside the uterus.
Eggs are common amonginvertebrates, includinginsects,spiders,[68]mollusks,[69] andcrustaceans.[70] Eggs deposited on land or in fresh water tend to have more yolk, which allows longer development in the egg before hatching. Eggs with little yoke hatch more rapidly into larval form that can seek out food. Some land invertebrates are viviparous, developing offspring within the body of the mother that are supplied nutrition by the host. Examples include thetsetse fly and someperipatus species.[71]
Parental care does occur in some invertebrate species, although rarely by the male; the addition of paternal care usually doesn't provide sufficient evolutionary advantage for it to evolve with any frequency. A counter-example is thedung beetle, where the male and female cooperate to bury balls of dung where the female can lay her eggs. Examples of invertebrates that provide parental care include thetreehopper andvelvet spider. Femalejumping spiders provide milk for their offspring.[72]
Many insect species and other invertebrate taxa are capable ofparthenogenesis, which is the production of offspring using an unfertilized egg. In thesubterranean termite, the queen produces new queen eggs via parthenogenesis but the soldiers and workers are created via sexual reproduction.[73] Unisexual reproduction is uncommon in vertebrates, but has been observed in some fish, reptile, and amphibian taxa.[74]
Evolution and structure
All sexually reproducing life, including both plants and animals, producesgametes.[75] 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 thezygote cell.[76] Inmulticellular organisms, the zygote subsequently divides in an organised manner into smaller more specialised cells (Embryogenesis), 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.[77]
A 2011 proposal suggests that thephylotypic animalbody plans originated 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.[78]
Theyolk component of the egg provides the nutrients needed for the growth of the embryo. More than half the proteins in egg yolk arephosphoglycoproteins, which are equivalent to milk proteins in mammals or storage proteins in plant seeds. The polypeptidevitellogenin (Vtg) is the major precursor of thelipoproteins andphosphoproteins that make up most of the protein content of yolk. It occurs in all egg-laying animals; the insect form is calledvitellin. The synthesis of yolk protein components occurs in the liver.[79] In an amniote egg, the yolk is surrounded by a membranousyolk sac that attaches to the embryo.[9]
The albumen, oregg white, is a clear liquid layer that surrounds the yolk. This material is hydrophilic and serves as a water reservoir for the embryo.[80] The predominant protein in egg whites isovalbumin, forming more than half the proteins by mass. The role of this protein is unknown.[81]Ovomucoid, which makes up 11% of thealbumen, is the primary eggallergen.[82]
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.[84]
Egg size and yolk
Vertebrate eggs 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.[7] This classification of eggs is based on the eggs ofchordates, though the basic principle extends to the wholeanimal kingdom. Within theegg cellcytoplasm, a uniform distribution of yolk is termedisolecithal, while an uneven distribution istelolecithal. Mammal eggs are isolecithal with small amounts of yolk, while bird and reptile eggs are telolecithal.[85]
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. Insponges andcnidarians, the dividing eggs develop directly into a simple larva, rather like amorula withcilia. In cnidarians, this stage is called theplanula, and either develops directly into the adult animals or forms new adult individuals through a process ofbudding.[86]
Microlecithal eggs require minimal yolk mass. Such eggs are found inflatworms,roundworms,annelids,bivalves,echinoderms, thelancelet and in most marinearthropods.[87] 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 possesses little if any yolk.[88]
Mesolecithal eggs have comparatively more yolk than the microlecithal eggs. The yolk is concentrated in one part of the egg (thevegetal pole), with thecell nucleus and most of thecytoplasm in the other (theanimal pole). The cell cleavage is uneven, and mainly concentrated in the cytoplasma-rich animal pole.[3]
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 inhagfish and somesnails.[4][87] 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,coelacanth and thesalamanders.[3]
Macrolecithal
A babytortoise begins to emerge "fully developed" from its macrolecithal egg.
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.[7] Macrolecithal eggs are only found in selected representatives of two groups:Cephalopods andvertebrates.[7][89]
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.[7] A portion of the yolk mass is still present as an external or semi-externalyolk sac at 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.[9]
In addition to bony fish and cephalopods, macrolecithal eggs are found incartilaginous fish,reptiles,birds andmonotreme mammals.[3] The eggs of thecoelacanths can 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.[90]
Egg-laying reproduction
Animals are commonly classified by their manner of reproduction, at the most general level distinguishing egg-laying (Latin.oviparous) from live-bearing (Latin.viviparous). French biologistThierry Lodé proposed a classification scheme that further divides the reproduction types according to the development that occurs before the offspring are expelled from the adult's body:[91]
Ovuliparity means the femalespawns unfertilized eggs (ova), which must then be externally fertilised. Ovuliparity is typical ofbony fish,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 notrophic (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 reptileAnguis fragilis, the sea horse (where zygotes are retained in the male's ventral "marsupium"), and the frogsRhinoderma darwinii (where the eggs develop in the vocal sac) andRheobatrachus (where the eggs develop in the stomach).
Histotrophic viviparity means embryos develop in the female'soviducts but obtain nutrients by consuming other ova, zygotes or sibling embryos (oophagy oradelphophagy). Thisintra-uterine cannibalism occurs in some sharks and in the black salamanderSalamandra atra.Marsupials excrete a "uterine milk" supplementing the nourishment from the yolk sac.[92]
Hemotrophic viviparity is where nutrients are provided from the female's blood through a designated organ. This most commonly occurs through aplacenta, found inmost mammals. Similar structures are found in some sharks and in the lizardPseudomoia pagenstecheri.[93][94] In somehylid frogs, the embryo is fed by the mother through specializedgills.[95]
The term hemotrophic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.[96]
Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia.[97] Popular choices for egg consumption are chicken, duck, roe, and caviar,[98] but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.[99]
According to theKashrut, that is the set of Jewishdietary laws,kosher food may be consumed according tohalakha (Jewish law).Eggs are consideredpareve (neither meat nor dairy) despite being an animal product and can be mixed with either milk or kosher meat.[100]
Vaccine manufacture
Preparation of measles vaccine at the Tirana (Albania) Institute of Hygiene and Epidemiology. Two technicians, wearing surgical gowns, are making small openings in eggs. (Photo by WHO)
ChocolateEaster eggs hidden as part of an egg hunt
Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring increation myths.[104]Egg decoration is a common practice in many cultures worldwide. Christians viewEaster eggs as symbolic of the resurrection of Jesus Christ.[105]
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.[106] Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken.[107]
Egg collecting was a popular hobby in some cultures, includingEuropean Australians. Traditionally, the embryo would be removed before a collector stored the egg shell.[108]
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.[109] 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,[110] and the collection in Western Australia Museum has been archived in a gallery.[111] 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.[112]
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