Egg
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An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell – a zygote. Within the vessel, an embryo is incubated until it has become an animal fetus that can survive on its own, at which point the animal hatches. Reproductive structures similar to the egg in other kingdoms are termed "spores", or in spermatophytes "seeds", or in gametophytes "egg cells".
Most arthropods, vertebrates (excluding live-bearing mammals), and mollusks lay eggs, although some, such as scorpions, do not. 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.
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 of decoration. 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 for infectious diseases.
Eggs of different animal groups
The largest recorded egg is from a whale shark and was Template:Convert in size.<ref>Template:Cite web</ref> Whale shark eggs typically hatch within the mother. At Template:Convert and up to Template:Convert, the ostrich egg is the largest egg of any living bird,<ref name="Khanna2005">Template:Cite book</ref>Template:Rp though the extinct elephant bird and some non-avian dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg, which measures between Template:Convert long and weighs half of a gram (around 0.02 oz).<ref name="Khanna2005"/>Template:Rp Some eggs laid by reptiles and most fish, amphibians, insects, and other invertebrates can be even smaller.
Several major groups of animals typically have readily distinguishable eggs.
| Class | Types of eggs | Development |
|---|---|---|
| Jawless fish | Mesolecithal eggs, especially large in hagfish<ref name=Hildebrand/> | Larval stage in lampreys, direct development in hagfish.<ref name=Gorbman/><ref>Template:Cite book</ref>Template:Page needed |
| Cartilaginous fish | Macrolecithal eggs with egg capsule<ref name=Hildebrand/> | Direct development, viviparity in some species<ref>Template:Cite book</ref>Template:Page needed |
| Bony fish | Macrolecithal eggs, small to medium size, large eggs in the coelacanth<ref name="Romer & Parson"/> | Larval stage, ovovivipary in some species.<ref>Template:Cite book</ref> |
| Amphibians | Medium-sized mesolecithal eggs in all species.<ref name="Romer & Parson"/> | Tadpole stage, direct development in some species.<ref name="Romer & Parson"/> |
| Reptiles | Large macrolecithal eggs, develop independent of water.<ref name="Stewart J. R. 1997"/> | Direct development, some ovoviviparious |
| Birds | Large to very large macrolecithal eggs in all species, develop independent of water.<ref name=Hildebrand/> | The young more or less fully developed, no distinct larval stage. |
| Mammals | Macrolecithal eggs in monotremes and marsupials, extreme microlecithal eggs in placental mammals.<ref name=Hildebrand/> | Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals. |
Fish and amphibian eggs
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The most common reproductive strategy for fish is known as oviparity,<ref name=Schreiber_2023>Template:Cite book</ref> in which the female lays undeveloped eggs that are externally fertilized by a male.<ref name=Diana_Höök_2023/> 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.<ref name=Duffy-Anderson_et_al_2024>Template:Cite book</ref>
A few fish, notably the rays and most sharks use ovoviviparity 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 as intrauterine cannibalism.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>
In certain scenarios, some fish such as the hammerhead shark and reef shark are viviparous, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment.<ref>Template:Cite book</ref>
The eggs of fish and amphibians (anamniotes) are jellylike.<ref name=Bonnan_2016>Template:Cite book</ref> Cartilaginous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development.<ref>Template:Cite book</ref> Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them.<ref name=Diana_Höök_2023>Template:Cite book</ref> These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water,<ref name=Bonnan_2016/> or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina.<ref>Template:Cite book</ref>
Amniote eggs and embryos
Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or embryos, including an amniotic membrane.<ref>Template:Cite book</ref> (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 include reptiles (including dinosaurs and their descendants, birds) and mammals.<ref name=Hayssen_Orr_2017>Template:Cite book</ref>
Reptile eggs are leathery for snakes and the majority of lizards, while turtles have a calcareous 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.<ref>Template:Cite book</ref> 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.<ref>Template:Cite book</ref> Not all reptiles lay eggs; some are viviparous ("live birth"). This adaptation may have allowed reptiles to inhabit new habitats, especially in colder climates.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
Among mammals, early extinct species were found to lay eggs, and was probably the ancestral state.<ref name=Hayssen_Orr_2017/> Platypuses and two genera of echidna (spiny anteaters) are Australian monotremes, the only order of extant egg-laying mammal.<ref>Template:Cite journal</ref> Marsupial and placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.<ref name=Hayssen_Orr_2017/>
Bird eggs
Template:Main Bird eggs are laid by females and incubated for a time that varies according to the species;<ref>Template:Cite book</ref> normally a single young hatches from each egg. Twin yolk eggs have been observed in domestic fowl, but this results in low hatchability.<ref>Template:Cite journal</ref> One case of twin geese has been observed to hatch from an elongated egg.<ref>Template:Cite journal</ref> Average clutch sizes range from one (as in condors<ref>Template:Cite journal</ref>) to about 17–24 (the grey partridge<ref>Template:Cite journal</ref>). It is rare for a bird to lay eggs when not fertilized,<ref>Template:Cite book</ref> known as parthenogenesis. One exception is the domestic hen; it is not uncommon for pet owners to find their lone bird nesting on a clutch of unfertilized eggs,<ref>Template:Cite book</ref> which are sometimes called wind-eggs.<ref>Template:Cite book</ref>
Shell
Bird eggs have a hard shell made of calcium carbonate with a 5% organic matrix. This resilient external surface prevents desiccation of the contents, limits mechanical damage, and protects against microbes, all while allowing the exchange of gas with the surrounding atmosphere.<ref>Template:Cite journal</ref> They vary in thickness from paper thin up to Template:Val in ostriches, and typically form Template:Val of the egg's weight.<ref name=Gill_2007/> Bird eggshells are diverse in appearance and structure.<ref name=Gill_2007>Template:Cite book</ref> For example:
- cormorant eggs are rough and chalky<ref>Template:Cite book</ref>
- tinamou eggs are shiny<ref name=Gill_2007/>
- duck eggs are oily and waterproof<ref name=Gill_2007/>
- cassowary eggs are heavily pitted<ref name=Gill_2007/>
- jacanas eggs appear lacquered<ref name=Gill_2007/>
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.<ref>Template:Cite book</ref> The domestic hen's egg has around 7000 pores.<ref>Template:Cite web</ref>
Some bird eggshells have a coating of vaterite spherules, which is a rare polymorph of calcium carbonate. In Greater Ani Crotophaga 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.<ref>Template:Cite journal</ref>
Shape
Bird egg shapes are ovoid and axisymmetrical in form, but vary by ellipticity and asymmetry depending on the bird species. Thus, the brown boobook species has a nearly spherical shell, the maleo egg is highly ellipsoidal, and the least 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.<ref>Template:Cite journal</ref><ref>Template:Cite news</ref>
Cliff-nesting birds often have highly conical 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 nearly spherical eggs.<ref>Template:Cite journal</ref>
Colours
The default colour of avian eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce coloured eggs. The colour comes from pigments deposited on top of the calcium carbonate base; biliverdin and its zinc chelate, and bilirubin, give a green or blue ground colour, while protoporphyrin IX produces reds and browns as a ground colour or as spotting.<ref name="ABCN">Template:Cite news</ref><ref>Template:Cite journal</ref><ref name=Kilner_2006/> Shell colours are secreted by the same oviduct 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.<ref name=Leahy_2021>Template:Cite book</ref>
Non-passerines typically have white eggs,<ref name=Attard_et_al_2023>Template:Cite journal</ref> except in some ground-nesting groups such as the Charadriiformes,<ref name=Attard_et_al_2023/> sandgrouse,<ref>Template:Cite journal</ref> and common terns,<ref>Template:Cite journal</ref> where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg.<ref>Template:Cite book</ref> 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.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref> Later eggs in a clutch are more spotted than early ones as the female's pigment glands become depleted.<ref name=Kilner_2006>Template:Cite journal</ref>
Within the common cuckoo lineage, the colour of individual eggs is genetically influenced, and appears to be inherited through the mother only. This suggests that the gene responsible for pigmentation is on the sex-determining W chromosome (female birds are WZ, males ZZ). However, egg colour in other species is most likely inherited from both parents.<ref>Template:Cite journal</ref> For chickens, egg colour appears determined from the hen's genome, diet, and stress factors like disease.<ref>Template:Cite web</ref> With American robins, there is some evidence that the brightness of the egg colouration may influence male parental care of the nestlings.<ref name=English_Montgomerie_2011>Template:Cite journal</ref>
Evolutionary factors can drive egg colouration, such as predation selecting for cryptic colouration, or colourful eggs possibly being used to coerce males into providing additional care during incubation – the blackmail hypothesis.<ref>Template:Cite journal</ref> For avian species that play host to brood 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 colouration evolutionary arms race between the host and parasite.<ref name=Stoddard_et_al_2017>Template:Cite journal</ref> In species such as the common 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.<ref>Template:Cite journal</ref>
Yolks of birds' eggs are yellow from carotenoids, it is affected by their living conditions and diet.<ref name="ABCN"/>
Predation
Many animals feed on eggs. For example, principal predators of the black oystercatcher's eggs include raccoons, skunk, mink, river and sea otters, gulls, crows and foxes.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref> The stoat (Mustela erminea) and long-tailed weasel (M. frenata) steal ducks' eggs.<ref>Template:Cite journal</ref> Snakes of the genera Dasypeltis and Elachistodon specialize in eating eggs.<ref>Template:Cite journal</ref>
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 being obligate parasites.<ref name=Stoddard_et_al_2017/> In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick.<ref>Template:Cite journal</ref> Brood parasites include the cowbird, black-headed duck, cuckoo-finch, and three Old World cuckoo species.<ref name=Stoddard_et_al_2017/>
Mammalian eggs
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.<ref>Template:Cite book</ref>
In placental mammals, there are two types of placenta: the yolk sac and the chorioallantoic. 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 the umbilical cord.<ref>Template:Cite book</ref> Receiving nutrients from the mother, the fetus completes the development while inside the uterus.
Invertebrate eggs
Eggs are common among invertebrates, including insects, spiders,<ref>Template:Cite book</ref> mollusks,<ref>Template:Cite book</ref> and crustaceans.<ref>Template:Cite book</ref> 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 the tsetse fly and some peripatus species.<ref>Template:Cite book</ref>
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 the dung 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 the treehopper and velvet spider. Female jumping spiders provide milk for their offspring.<ref>Template:Cite book</ref>
Many insect species and other invertebrate taxa are capable of parthenogenesis, which is the production of offspring using an unfertilized egg. In the subterranean termite, the queen produces new queen eggs via parthenogenesis but the soldiers and workers are created via sexual reproduction.<ref>Template:Cite encyclopedia</ref> Unisexual reproduction is uncommon in vertebrates, but has been observed in some fish, reptile, and amphibian taxa.<ref>Template:Cite book</ref>
Evolution and structure
All sexually reproducing life, including both plants and animals, produces gametes.<ref>Template:Cite web</ref> The male gamete cell, sperm, is usually motile whereas the female gamete cell, the ovum, is generally larger and sessile. The male and female gametes combine to produce the zygote cell.<ref>Template:Cite book</ref> In multicellular organisms, the zygote subsequently divides in an organised manner into smaller more specialised cells (Embryogenesis), so that this new individual develops into an embryo. 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.<ref>Template:Cite book</ref>
A 2011 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.<ref name=Newman>Template:Cite journal</ref>
The yolk component of the egg provides the nutrients needed for the growth of the embryo. More than half the proteins in egg yolk are phosphoglycoproteins, which are equivalent to milk proteins in mammals or storage proteins in plant seeds. The polypeptide vitellogenin (Vtg) is the major precursor of the lipoproteins and phosphoproteins that make up most of the protein content of yolk. It occurs in all egg-laying animals; the insect form is called vitellin. The synthesis of yolk protein components occurs in the liver.<ref>Template:Cite journal</ref> In an amniote egg, the yolk is surrounded by a membranous yolk sac that attaches to the embryo.<ref name="Stewart J. R. 1997"/>
The albumen, or egg white, is a clear liquid layer that surrounds the yolk. This material is hydrophilic and serves as a water reservoir for the embryo.<ref>Template:Cite book</ref> The predominant protein in egg whites is ovalbumin, forming more than half the proteins by mass. The role of this protein is unknown.<ref>Template:Cite book</ref> Ovomucoid, which makes up 11% of the albumen, is the primary egg allergen.<ref>Template:Cite journal</ref>
Formation
The cycle of the egg's formation is started by the gamete ovum being released (ovulated) and egg formation being started. Within the oviduct, the albumen, shell membranes, and outer shell can then be applied.<ref>Template:Cite book</ref> The finished egg is then ovipositioned and eventual egg incubation can start.
Scientific classifications
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.<ref>Template:Cite book</ref>
Egg size and yolk
Vertebrate eggs can be classified by the relative amount of yolk. Simple eggs with little yolk are called microlecithal, medium-sized eggs with some yolk are called mesolecithal, and large eggs with a large concentrated yolk are called macrolecithal.<ref name="Romer & Parson">Template:Cite book</ref> This classification of eggs is based on the eggs of chordates, though the basic principle extends to the whole animal kingdom. Within the egg cell cytoplasm, a uniform distribution of yolk is termed isolecithal, while an uneven distribution is telolecithal. Mammal eggs are isolecithal with small amounts of yolk, while bird and reptile eggs are telolecithal.<ref>Template:Cite book</ref>
Microlecithal
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 sponges and cnidarians, the dividing eggs develop directly into a simple larva, rather like a morula with cilia. In cnidarians, this stage is called the planula, and either develops directly into the adult animals or forms new adult individuals through a process of budding.<ref>Template:Cite journal</ref>
Microlecithal eggs require minimal yolk mass. Such eggs are found in flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods.<ref name="Barns">Template:Cite book</ref> 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 into larvae, 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.<ref>Template:Cite book</ref>
Mesolecithal
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.<ref name=Hildebrand>Template:Cite book</ref>
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.<ref name=Gorbman>Template:Cite journal</ref><ref name="Barns"/> 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.<ref name=Hildebrand/>
Macrolecithal
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.<ref name="Romer & Parson"/> Macrolecithal eggs are only found in selected representatives of two groups: Cephalopods and vertebrates.<ref name="Romer & Parson"/><ref>Template:Cite conference</ref>
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.<ref name="Romer & Parson"/> A portion of the yolk mass is still present as an external or semi-external yolk sac at hatching in many groups. This form of fetal development is common in bony 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, see amniote.<ref name="Stewart J. R. 1997">Template:Cite book</ref>
In addition to bony fish and cephalopods, macrolecithal eggs are found in cartilaginous fish, reptiles, birds and monotreme mammals.<ref name=Hildebrand/> The eggs of the coelacanths can reach a size of Template:Cvt in diameter, and the young go through full development while in the uterus, living on the copious yolk.<ref>Template:Cite journal</ref>
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 biologist Thierry 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:<ref name=Lodé_2012>Template:Cite journal</ref>
- Ovuliparity means the female spawns unfertilized eggs (ova), which must then be externally fertilised. Ovuliparity is typical of bony 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 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 oviducts but obtain nutrients by consuming other ova, zygotes or sibling embryos (oophagy or adelphophagy). This intra-uterine cannibalism occurs in some sharks and in the black salamander Salamandra atra. Marsupials excrete a "uterine milk" supplementing the nourishment from the yolk sac.<ref>Template:Cite book</ref>
- 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In some hylid frogs, the embryo is fed by the mother through specialized gills.<ref>Template:Cite book</ref>
The term hemotrophic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.<ref>Template:Cite web</ref>
Human use
Food
Template:MainTemplate:Cookbook Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia.<ref>Template:Cite book</ref> Popular choices for egg consumption are chicken, duck, roe, and caviar,<ref>Template:Cite book</ref> but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.<ref>Template:Cite book</ref>
Eggs and Kashrut
Template:See also According to the Kashrut, that is the set of Jewish dietary laws, kosher food may be consumed according to halakha (Jewish law). Eggs are considered pareve (neither meat nor dairy) despite being an animal product and can be mixed with either milk or kosher meat.<ref>Template:Cite web</ref>
Vaccine manufacture
Many vaccines for infectious diseases are produced in fertile chicken eggs.<ref>Template:Cite web</ref> The basis of this technology was the discovery in 1931 by Alice Miles Woodruff and Ernest William Goodpasture at Vanderbilt University that the rickettsia and viruses that cause a variety of diseases will grow in chicken embryos.<ref>Template:Cite journal</ref> This enabled the development of vaccines against influenza, chicken pox, smallpox, yellow fever, typhus, Rocky mountain spotted fever and other diseases.<ref>Template:Cite book</ref>
Culture
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Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths.<ref>Template:Cite web</ref> Egg decoration is a common practice in many cultures worldwide. Christians view Easter eggs as symbolic of the resurrection of Jesus Christ.<ref>Template:Cite news</ref>
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.<ref>Template:Cite book</ref> Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken.<ref>Template:Cite news</ref>
Collecting
Template:Main Egg collecting was a popular hobby in some cultures, including European Australians. Traditionally, the embryo would be removed before a collector stored the egg shell.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref> 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,<ref>Template:Cite web</ref> and the collection in Western Australia Museum has been archived in a gallery.<ref>Template:Cite web</ref> 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.<ref>Template:Cite news</ref>
See also
- List of egg topics
- Animal shell
- Butterfly eggs
- Egg white
- Fossil egg
- Haugh unit
- Oology
- Oviparous
- Trophic egg
- Organic egg production
References
External links
Template:Eggs Template:Birds Template:Portal bar Template:Authority control