Egg (biology)
   HOME

TheInfoList




An egg is the organic vessel containing the
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
in which an
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
develops until it can survive on its own, at which point the animal hatches. An egg results from
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
of an
egg cell The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduct ...
. Most
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s,
vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...

vertebrates
(excluding
live-bearing mammal Theria (; Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million a ...
s), and
mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil species is e ...

mollusks
lay eggs, although some, such as
scorpion Scorpions are predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including hum ...

scorpion
s, do not.
Reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

Reptile
eggs,
bird egg Bird eggs are laid by the female Female (symbol: ♀) is the sex Sex is either of two divisions, typically male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger femal ...

bird egg
s, and
monotreme Monotremes are prototherian mammals of the order Monotremata. They are one of the three main groups of living mammals, along with placentals (Eutheria) and marsupials (Metatheria). Monotremes are typified by structural differences in their brai ...
eggs are laid out of water and are surrounded by a protective
shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard out ...
, 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 An egg tooth is a temporary, sharp projection present on the bill Bill(s) may refer to: Common meanings * Banknote A banknote (often known as a bill (in the US and Canada), paper money, or simply a note) is a type of negotiable instrumen ...
they use to crack, pip, or break the eggshell or covering. The largest recorded egg is from a
whale shark The whale shark (''Cebuano: Taweki'')(''Rhincodon typus'') is a slow-moving, filter-feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passin ...
and was in size. Whale shark eggs typically hatch within the mother. At and up to , the
ostrich ''Struthio'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circums ...

ostrich
egg is the largest egg of any living bird, though the extinct
elephant bird Elephant birds are members of the extinct Extinction is the termination of a kind of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bi ...

elephant bird
and some
non-avian dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge ...
s laid larger eggs. The
bee hummingbird#REDIRECT Bee hummingbird {{Redirect category shell, 1= {{R from move {{R from other capitalisation ...
produces the smallest known bird egg, which weighs half of a gram (around 0.02 oz). Some eggs laid by reptiles and most fish, amphibians, insects, and other
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s can be even smaller. Reproductive structures similar to the egg in other
kingdom Kingdom may refer to: Monarchy * A type of monarchy * A realm ruled by: **A king, during the reign of a male monarch **A queen regnant, during the reign of a female monarch Taxonomy * Kingdom (biology), a category in biological taxonomy Arts a ...
s are termed "
spore In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...
s," or in
spermatophyte The spermatophytes (; ), also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to E ...
s "
seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positi ...

seed
s," or in
gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert ...
s "egg cells".


Eggs of different animal groups

Several major groups of animals typically have readily distinguishable eggs.


Fish and amphibian eggs

The most common reproductive strategy for
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
is known as
oviparity Oviparous animals are female animals that lay their egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excludi ...
, 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 Cod is the common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is sometim ...

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
ray Ray may refer to: Science and mathematics * Ray (geometry) In geometry, the notion of line or straight line was introduced by ancient mathematicians to represent straight objects (i.e., having no curvature) with negligible width and depth. ...
s and most
shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified withi ...

shark
s use
ovoviviparity Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animal Animals (also called Met ...
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 Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecology, ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well docume ...
. In certain scenarios, some fish such as the
hammerhead shark The hammerhead sharks are a group of shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused ...

hammerhead shark
and reef shark are
viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...
, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment. The eggs of fish and
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s are 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
Chiromantis xerampelina
''.


Bird eggs

Bird eggs are laid by females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one (as in condors) to about 17 (the grey partridge). Some birds lay eggs even when not fertilized (e.g. chicken, 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.


Colours

The default color of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce colored eggs. The pigment biliverdin and its zinc chelate give a green or blue ground color, and protoporphyrin produces reds and browns as a ground color or as spotting. 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 brood parasite, 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 gene responsible for pigmentation is on the sex-determining W 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 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.


Shell

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 Tiny pores in bird eggshells allow the embryo to breathe. The chicken, domestic hen's egg has around 7000 pores. 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.


Shape

Most bird eggs have an Oval (geometry), oval shape, with one end rounded and the other more pointed. This shape results from the egg being forced through the oviduct. 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. 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 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.


Predation

Many animals feed on eggs. For example, principal predators of the American black oystercatcher, black oystercatcher's eggs include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes. The stoat (''Mustela erminea'') and long-tailed weasel (''M. frenata'') steal ducks' eggs. Snakes of the genera ''Dasypeltis'' and ''Elachistodon'' specialize in eating eggs. Brood parasitism occurs 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 the cowbirds and many Old World cuckoos.


Various examples

File:Egg125o.png, An average whooping crane egg is long and weighs File:Oystercatcher Eggs Norway.jpg, Eurasian oystercatcher eggs camouflaged in the nest File:Senegal egg 10s06.JPG, Egg of a senegal parrot, a bird that nests in tree holes, on a grid File:Comparison of eggs by Zureks.jpg, Eggs of
ostrich ''Struthio'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circums ...

ostrich
, emu, Kiwi (bird), kiwi and chicken File:Finch Egg.jpg, Finch egg next to Dime (United States coin), American dime File:Ouă.jpg, Eggs of duck, goose, guineafowl and chicken File:Various_eggs.JPG, Eggs of
ostrich ''Struthio'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circums ...

ostrich
, southern cassowary, cassowary, chicken, flamingo, pigeon and common blackbird, blackbird File:Emu Egg.JPG, Egg of an emu File:Eggs from a chicken and great tit.jpg, Egg from a chicken compared to a 1 euro coin, great tit egg and a corn grain File:Vogelnest Bodenbrüter.jpg, Bird nest with brown marble, marbling eggs of a European robin, robin


Amniote eggs and embryos

Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
s, including an amniotic membrane. Amniotes include reptiles (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 are viviparous ("live birth"). Dinosaurs laid eggs, some of which have been preserved as petrified fossils. Among mammals, early extinct species laid eggs, as do platypuses and echidnas (spiny anteaters). Platypuses and two genera of echidna are Australian monotremes. Marsupial and placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.


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. In placental mammals, the egg itself is void of yolk, but develops an umbilical cord from structures that in reptiles would form the yolk sac. Receiving nutrients from the mother, the fetus completes the development while inside the uterus.


Invertebrate eggs

Eggs are common among
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s, including insects, spiders, mollusks, and crustaceans.


Evolution and structure

All sexually reproducing life, including both plants and animals, produces gametes. The male gamete cell, sperm, is usually motile whereas the female gamete cell, the ovum, is generally larger and wikt:sessile, sessile. The male and female gametes combine to produce the
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
cell. In multicellular organisms the zygote subsequently divides in an organised manner into smaller more specialised cells, so that this new individual develops into an
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

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. A recent proposal suggests that the phylotype, phylotypic animal body plans originated in cell aggregates before the existence of an egg stage of developmental biology, development. Eggs, in this view, were later evolutionary emergence, innovations, selected for their role in ensuring genetic uniformity among the cells of incipient multicellular organisms.


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.


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''.Alfred Romer, Romer, A. S. & Parsons, T. S. (1985): ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia. This classification of eggs is based on the eggs of chordata, chordates, though the basic principle extends to the whole animalia, animal kingdom.


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 cilium, 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. Microlecithal eggs require minimal yolk mass. Such eggs are found in flatworms, roundworms, annelids, bivalves, Echinodermata, echinoderms, the lancelet and in most marine
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s.Barns, R.D. (1968): Invertebrate Zoology. W. B. Saunders Company, Philadelphia. 743 pages 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 is reduced in size to essentially a naked egg cell.


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.Hildebrand, M. & Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. ''John Wiley & Sons, Inc''. New York City 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.


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. 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. 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.Stewart J. R. (1997): ''Morphology and evolution of the egg of oviparous amniotes''. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291–326. London: Academic Press. In addition to bony fish and cephalopods, macrolecithal eggs are found in Chondrichthyes, cartilaginous fish, reptiles, birds and
monotreme Monotremes are prototherian mammals of the order Monotremata. They are one of the three main groups of living mammals, along with placentals (Eutheria) and marsupials (Metatheria). Monotremes are typified by structural differences in their brai ...
mammals. The eggs of the coelacanths can reach a size of in diameter, and the young go through full development while in the uterus, living on the copious yolk.


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''). 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 Spawn (biology), 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. *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 placentalia, most mammals. Similar structures are found in some sharks and in the lizard ''Pseudomoia pagenstecheri''. In some Hylidae, 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.


Human use


Food

Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by mankind 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 Kashrut, that is the set of Jewish Taboo food and drink, dietary laws, kosher food may be consumed according to ''halakha'' (Jewish law). Kosher meat and milk (or derivatives) cannot be mixed () or stored together. egg (food), Eggs are considered ''pareve'' (neither meat nor dairy) despite being an animal product and can be mixed with either milk or kosher meat. Mayonnaise, for instance, is usually marked "pareve" despite by definition containing egg.


Vaccine manufacture

Many vaccines for infectious diseases are produced in fertile chicken eggs. 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. This enabled the development of vaccines against influenza, chicken pox, smallpox, yellow fever, typhus, Rocky mountain spotted fever and other diseases.


Culture

Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths. Egg decorating, Egg decoration is a common practice in many cultures worldwide. Christians view Easter eggs as 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 an Easter 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 a dancing egg is held during the feast of Corpus Christi in Barcelona and other 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.


Collecting

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. 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.


Gallery

File:Emperor Gum Moth eggs2.jpg, Insect eggs, in this case those of the Opodiphthera eucalypti, Emperor gum moth, are often laid on the underside of leaves. File:Clupeaharenguskils2.jpg, Fish eggs, such as these herring eggs are often transparent and fertilized after laying. File:Skate egg case (Raja binoculata) 01.jpg, Skate (fish), Skates and some
shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified withi ...

shark
s have a uniquely shaped egg case called a mermaid's purse. File:Éclosion tortue d'Hermann.JPG, A ''Testudo hermanni'' emerging fully developed from a reptilian egg. File:S mekongi eggR.jpg, A ''Schistosoma mekongi'' egg. File:Huffmanela hamo eggs (Microscope) 1D.JPG, Eggs of ''Huffmanela hamo'', a nematode parasite in a fish File:Parasite140080-fig3 Gastrointestinal parasites in seven primates of the Taï National Park - Helminths.png, Eggs of various parasites (mainly nematodes) from wild primates


See also

*List of egg topics *Animal shell *External morphology of Lepidoptera#Egg, Butterfly eggs *Egg white *Fossil egg *Haugh unit *Oology *Oval *Ovary *Ovulation *Oviparous *Trophic egg


References

{{DEFAULTSORT:Egg (Biology) Eggs, Animal reproductive system Oology Bird breeding Aviculture