{{short description|Organic vessel in which an embryo first begins to develop}}
{{About|biological eggs|eggs as food|Eggs as food|other uses|Egg (disambiguation)}}
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{{More citations needed|date=February 2019}}
[[File:Oeufs002b.jpg|Eggs of various [[bird]]s, [[turtle|a reptile]], various [[cartilaginous fish]], a [[cephalopod|cuttlefish]] and various [[butterflies and moths]]. (Click on image for key)|right|thumb]]
[[File:Chicken egg diagram.svg|thumb|Diagram of a chicken egg in its ninth day. Membranes: allantois, chorion, amnion, and vitellus/yolk.]]
[[File:Six eggs views from the top on a white background.jpg|thumb|Six commercial eggs {{mdash}} view from the top against a white background]]

An '''egg''' is an organic vessel grown by an animal to carry a possibly [[fertilization|fertilized]] [[egg cell]] (a [[zygote]]) and to [[egg incubation|incubate]] from it an [[embryo]] within the egg until the embryo has become an animal [[fetus]] that can survive on its own, at which point the animal hatches.

Most [[arthropod]]s, [[vertebrates]] (excluding [[live-bearing mammal]]s), and [[Mollusca|mollusks]] lay eggs, although some, such as [[scorpion]]s, do not.

[[Reptile]] eggs, [[bird egg]]s, and [[monotreme]] eggs are laid out of water and are surrounded by a protective [[eggshell|shell]], either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches, i.e., breaks out of the egg's shell. Some embryos have a temporary [[egg tooth]] they use to crack, pip, or break the eggshell or covering.

The largest recorded egg is from a [[whale shark]] and was {{convert|30|x|14|x|9|cm|abbr=on}} in size.<ref>{{cite web |title=Whale Shark – Cartilaginous Fish |url=http://seaworld.org/en/animal-info/animal-bytes/cartilaginous-fish/whale-shark/ |publisher=[[SeaWorld]] Parks & Entertainment |access-date=27 June 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140609153809/http://seaworld.org/en/animal-info/animal-bytes/cartilaginous-fish/whale-shark/ |archive-date=9 June 2014 }}</ref> Whale shark eggs typically hatch within the mother. At {{convert|1.5|kg|abbr=on}} and up to {{convert|17.8|x|14|cm|abbr=on}}, the [[ostrich]] egg is the largest egg of any living bird,<ref name="Khanna2005">{{cite book|last1=Khanna |first1=D.R. |year=2005 |title=Biology of Birds |url=https://books.google.com/books?id=fDblIChi7KwC&pg=PA130 |location=New Delhi, India |publisher=Discovery Publishing House |isbn=978-81-7141-933-3 |url-status=live |archive-url=https://web.archive.org/web/20160510084347/https://books.google.com/books?id=fDblIChi7KwC&pg=PA130|archive-date=10 May 2016 }}</ref>{{rp|130}} though the extinct [[elephant bird]] and some [[non-avian dinosaur]]s laid larger eggs. The [[bee hummingbird]] produces the smallest known bird egg, which measures between {{convert|6.35-11.4|mm|in}} long and weighs half of a gram (around 0.02&nbsp;oz).<ref name="Khanna2005"/>{{rp|132}} Some eggs laid by reptiles and most fish, amphibians, insects, and other [[invertebrate]]s can be even smaller.

Reproductive structures similar to the egg in other [[kingdom (biology)|kingdom]]s are termed "[[spore]]s", or in [[spermatophyte]]s "[[seed]]s", or in [[gametophyte]]s "egg cells".

== Eggs of different animal groups ==
{{Further|Egg cell}}

Several major groups of animals typically have readily distinguishable eggs.

{| class="wikitable"
|+ Overview of eggs from various animals
! [[Class (biology)|Class]]
! Types of eggs
! Development
|-
| [[Agnatha|Jawless fish]]
| Mesolecithal eggs, especially large in [[hagfish]]<ref name=Hildebrand/>
| [[Larval]] stage in [[lamprey]]s, direct development in hagfish.<ref name=Gorbman/><ref>{{cite book |last1=Hardisty |first1=M. W. |last2=Potter |first2=I. C. |year=1971 |title=The Biology of Lampreys |url=https://books.google.com/books?id=YaPwAAAAMAAJ |volume=2 |edition=1st |location=New York, USA |publisher=Academic Press Inc. |isbn=0-12-324801-9}}</ref>{{Page needed|date=January 2023}}
|-
| [[Chondrichthyes|Cartilaginous fish]]
| Macrolecithal eggs with [[Egg case (Chondrichthyes)|egg capsule]]<ref name=Hildebrand/>
| Direct development, [[viviparity]] in some species<ref>{{cite book|last1=Compagno |first1=Leonard J. V. |year=1984 |title=Sharks of the World: An annotated and illustrated catalogue of shark species known to date |publisher=Food and Agriculture Organization of the United Nations |isbn=92-5-104543-7 |oclc=156157504}}</ref>{{Page needed|date=January 2023}}
|-
| [[Osteichthyes|Bony fish]]
| Macrolecithal eggs, small to medium size, large eggs in the [[coelacanth]]<ref name="Romer & Parson"/>
| [[Larva]]l stage, [[Ovoviviparity|ovovivipary]] in some species.<ref>{{cite book |last=Peter |first=Scott |date=1997 |title=Livebearing Fishes |location=Blacksburg, Virginia, USA |publisher=Tetra Press |page=13 |isbn=1-56465-193-2}}</ref>
|-
| [[Amphibian]]s
| 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 [[monotreme]]s and [[marsupial]]s, extreme microlecithal eggs in [[placental mammals]].<ref name=Hildebrand/>
| Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals.
|}

{{anchor|Fish eggs|Amphibian eggs}}
=== Fish and amphibian eggs ===
{{redirect|Fish egg|fish eggs as food|Roe}}
{{see also|Ichthyoplankton|Spawn (biology)}}
{{multiple image
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 | width     = 220
 | image1    = Salmoneggskils.jpg
 | alt1      = 
 | caption1  = Salmon eggs in different stages of development. In some only a few cells grow on top of the [[yolk]], in the lower right the blood vessels surround the yolk and in the upper left the black eyes are visible.
 | image2    = Fish Egg Diagram (1).jpg
 | alt2      = 
 | caption2  = Diagram of a fish egg: A. [[vitelline membrane]] B. [[chorion]] C. [[egg yolk|yolk]] D. oil globule E. [[perivitelline space]] F. [[embryo]]
}}
[[File:Salmonlarvakils.jpg|thumb|right|150px|[[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 for [[fish]] is known as [[oviparity]], in which the female lays undeveloped eggs that are externally fertilized by a male. Typically large numbers of eggs are laid at one time (an adult female [[cod]] can produce 4–6&nbsp;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 (fish)|ray]]s and most [[shark]]s 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 [[Cannibalism (zoology)#Intrauterine cannibalism|intrauterine cannibalism]].

In certain scenarios, some fish such as the [[hammerhead shark]] and [[reef shark (SIA)|reef shark]] are [[Viviparity|viviparous]], with the egg being fertilized and developed internally, but with the mother also providing direct nourishment.
[[File:RanaArvalisMatingPlusSpawn.jpg|thumb|[[Moor frog]] eggs utilize [[Glycan|glycans]] to form a hydrophilic jelly coat that protects the egg]]
The eggs of fish and [[amphibian]]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]]''.

=== Bird eggs ===
{{main|Bird egg}}
Bird eggs are laid by females and [[Incubate (bird)|incubated]] for a time that varies according to the species; a single young hatches from each egg. Average [[clutch (eggs)|clutch size]]s range from one (as in [[condor]]s) 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 ====
[[File:Uria aalge MHNT Box Rouzic.jpg|thumb|[[Guillemot]] eggs]]
The default colour of vertebrate eggs is the white of the [[calcium carbonate]] from which the shells are made, but some birds, mainly [[passerine]]s, produce coloured eggs. The colour comes from [[pigment]]s 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">{{cite news |last1=Rääbus |first1=Carol |date=18 February 2018 |title=The chemistry of eggshell colours |url=https://www.abc.net.au/news/2018-02-19/chemistry-of-eggshell-colour/9455660 |work=[[ABC News (Australia)|ABC News]] |publisher=[[Australian Broadcasting Corporation]] |access-date=21 January 2023}}</ref>

Non-passerines typically have white eggs, except in some ground-nesting groups such as the [[Charadriiformes]], [[sandgrouse]] and [[nightjar]]s, where camouflage is necessary, and some [[brood parasite|parasitic]] [[cuckoo]]s which have to match the passerine host's egg. Most passerines, in contrast, lay coloured eggs, even if there is no need of cryptic colors. However, some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid-state lubricant.<ref>{{cite book |last=Solomon |first=S.E. |editor-last1=Wells |editor-first1= R.G. |editor-last2=Belyarin |editor-first2=C.G. |year=1987 |chapter=Egg shell pigmentation |title=Egg Quality: Current Problems and Recent Advances |location=London |publisher=Butterworths |pages=147–157 }}</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>{{cite journal|last1=Gosler |first1=Andrew G. |last2=Higham |first2=James P. |last3=Reynolds |first3=S. James |title=Why are birds' eggs speckled? |year=2005 |journal=Ecology Letters |volume= 8|issue=10 |pages=1105–1113 |doi= 10.1111/j.1461-0248.2005.00816.x}}</ref>

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.

[[Yolk]]s of birds' eggs are yellow from [[carotenoid]]s, it is affected by their living conditions and diet.<ref name="ABCN"/>

==== 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.<ref>{{Cite web|url=https://www.sites.ext.vt.edu/virtualfarm/poultry/poultry_eggparts.html|archiveurl=https://web.archive.org/web/20161123092430/http://www.sites.ext.vt.edu/virtualfarm/poultry/poultry_eggparts.html|url-status=dead|title=The Parts of the Egg|archivedate=November 23, 2016|website=www.sites.ext.vt.edu}}</ref>

Some bird eggshells have a coating of [[vaterite]] [[spherule]]s, 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>{{cite journal |author1= Portugal, J. P. |author2= Bowen, J. | author3= Riehl, C. | year=2018| title= A rare mineral, vaterite, acts as a shock absorber in the eggshell of a communally nesting bird. | journal=Ibis| volume=160| pages=173–178| doi=10.1111/ibi.12527 | issue=1|url=http://oro.open.ac.uk/50473/1/ORO%2050473.pdf| doi-access=free}}</ref>

==== Shape ====
[[File:Oeuf.stl|thumb|A 3D model of an egg]]
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.{{Citation needed|date=December 2022}}. 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>{{cite journal |last1=Stoddard |first1=Mary Caswell |last2=Yong |first2=Ee Hou |last3=Akkaynak |first3=Derya |last4=Sheard |first4=Catherine |last5=Tobias |first5=Joseph A. |last6=Mahadevan |first6=L. |title=Avian egg shape: Form, function, and evolution |journal=Science |date=23 June 2017 |volume=356 |issue=6344 |pages=1249–1254 |doi=10.1126/science.aaj1945|pmid=28642430 |bibcode=2017Sci...356.1249S |hdl=10044/1/50092 |s2cid=11962022 |hdl-access=free }}</ref><ref>{{cite news|last1=Yong |first1=Ed |date=22 June 2017 |title=Why Are Bird Eggs Egg-Shaped? An Eggsplainer |url=https://www.theatlantic.com/science/archive/2017/06/why-are-bird-eggs-egg-shaped/531261/ |access-date=23 June 2017 |work=[[The Atlantic]] |url-status=live |archive-url=https://web.archive.org/web/20170624145039/https://www.theatlantic.com/science/archive/2017/06/why-are-bird-eggs-egg-shaped/531261/|archive-date=24 June 2017}}</ref>

Cliff-nesting birds often have highly [[cone|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 [[sphere|spherical]] eggs.<ref>{{cite journal |author=Yutaka Nishiyama |author-link=Yutaka Nishiyama |title=The Mathematics of Egg Shape |url=https://ijpam.eu/contents/2012-78-5/8/8.pdf |year=2012 |journal=International Journal of Pure and Applied Mathematics |volume=78 |number=5 |pages=679–689 }}</ref>

==== Predation ====
Many animals feed on eggs. For example, principal predators of the [[American black oystercatcher|black oystercatcher]]'s eggs include [[raccoon]]s, [[skunk]]s, [[mink]], river and sea [[otter]]s, [[gull]]s, [[crow]]s and [[fox]]es. The [[stoat]] (''Mustela erminea'') and [[long-tailed weasel]] (''M. frenata'') steal ducks' eggs. [[Snake]]s 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 [[cowbird]]s and many Old World [[cuckoo]]s.

==== Various examples ====
<gallery>
File:Egg125o.png|An average [[whooping crane]] egg is {{convert|102|mm|abbr=on}} long and weighs {{convert|208|g|oz|abbr=on}}
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 {{convert|1|cm|in|abbr=on}} grid
File:Comparison of eggs by Zureks.jpg|Eggs of [[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]], [[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]]
File:Spherical egg.jpg|A spherical chicken egg
</gallery>

=== Amniote eggs and embryos ===
[[File:Snapping turtle eggs md.jpg|thumb|right|[[Turtle]] eggs in a nest dug by a female common snapping turtle (''[[common snapping turtle|Chelydra serpentina]]'')]]
Like amphibians, [[amniote]]s are air-breathing [[vertebrate]]s, but they have complex eggs or [[embryo]]s, including an [[amniotic membrane]]. Amniotes include [[reptile]]s (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 [[platypus]]es and [[echidna]]s (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 [[echidna]]s) are macrolecithal eggs very much like those of [[reptile]]s. The eggs of [[marsupial]]s 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>Colbert, H.E & Morales, M. (1991): ''Evolution of the Vertebrates – A History of Backboned Animals Through Time''. 4. utgave. John Wiley & Sons inc, [[New York City]]. 470 pages {{ISBN|0-471-85074-8}}</ref>

In [[placental mammal]]s, 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 ===
[[File:Editing Image-Acanthodoris lutea laying eggs 2.jpg|[[Orange-peel doris]] (''Acanthodoris lutea''), a [[nudibranch]], in [[tide pool]] laying eggs|right|thumb]]
Eggs are common among [[invertebrate]]s, including [[insect]]s, [[spider]]s, [[mollusk]]s, and [[crustacean]]s.

==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]] 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]]. 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 plan]]s 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.<ref name=Newman>{{cite journal|last=Newman|first=S.A.|title=Animal egg as evolutionary innovation: a solution to the 'embryonic hourglass' puzzle|journal= Journal of Experimental Zoology Part B: Molecular and Developmental Evolution|date=2011|volume=316|issue=7|pages=467–483|doi=10.1002/jez.b.21417|pmid=21557469}}</ref>

==Formation==
The cycle of the egg's formation is started by the [[gamete]] [[ovum]] being released ([[ovulation|ovulated]]) and egg formation being started. The finished egg is then [[oviposition]]ed 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.

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

==== Microlecithal ====
[[File:Toxocara embryonated eggs.jpg|thumb|right|Microlecithal eggs from the [[Nematoda|roundworm]] ''[[Toxocara]]'']]
[[File:Paragonimus westermani 01.jpg|thumb|right|Microlecithal eggs from the [[flatworm]] ''[[Paragonimus westermani]]'']]
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 [[cnidaria]]ns, 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]].<ref>{{cite journal|last=Reitzel|first=A.M.|author2=Sullivan, J.C|author3=Finnery, J.R|title=Qualitative shift to indirect development in the parasitic sea anemone ''Edwardsiella lineata''|journal=Integrative and Comparative Biology|year=2006|volume=46|issue=6|pages=827–837|doi=10.1093/icb/icl032|pmid=21672788|doi-access=free}}</ref>

Microlecithal eggs require minimal yolk mass. Such eggs are found in [[flatworm]]s, [[roundworm]]s, [[annelid]]s, [[bivalve]]s, [[Echinodermata|echinoderms]], the [[lancelet]] and in most marine [[arthropod]]s.<ref name="Barns">Barns, R.D. (1968): Invertebrate Zoology. W. B. Saunders Company, Philadelphia. 743 pages</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 [[larva]]e, 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====
[[File:Frogspawn closeup.jpg|thumb|right|[[Frog]]spawn is 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>Hildebrand, M. & Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. ''John Wiley & Sons, Inc''. [[New York City]]</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 [[snail]]s.<ref name=Gorbman>{{cite journal|last=Gorbman|first=A.|title=Hagfish development|journal= Zoological Science|date=June 1997|volume=14|issue=3|pages=375–390|doi=10.2108/zsj.14.375|s2cid=198158310|doi-access=free}}</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 [[lamprey]]s, [[coelacanth]] and the [[salamander]]s.<ref name=Hildebrand/>

====Macrolecithal====
[[File:Tortoise-Hatchling.jpg|thumb|right|A baby [[tortoise]] 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.<ref name="Romer & Parson"/> Macrolecithal eggs are only found in selected representatives of two groups: [[Cephalopod]]s and [[vertebrate]]s.<ref name="Romer & Parson"/><ref>Nixon, M. & Messenger, J.B (eds) (1977): The Biology of Cephalopods. Symposium of the Zoological Society of London, pp 38–615</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">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.</ref>

In addition to bony fish and cephalopods, macrolecithal eggs are found in [[Chondrichthyes|cartilaginous fish]], [[reptile]]s, [[bird]]s and [[monotreme]] mammals.<ref name=Hildebrand/> The eggs of the [[coelacanth]]s can reach a size of {{cvt|9|cm}} in diameter, and the young go through full development while in the [[uterus]], living on the copious yolk.<ref>Fricke, H.W. & Frahm, J. (1992): Evidence for lecithotrophic viviparity in the living coelacanth. ''Naturwissenschaften'' no 79: pp. 476–479</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'').

These classifications are divided into more detail according to the development that occurs before the offspring are expelled from the adult's body. Traditionally:<ref>[[Thierry Lodé]] 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris</ref>
*'''Ovuliparity''' means the female [[Spawn (biology)|spawns]] unfertilized eggs (ova), which must then be externally fertilised. Ovuliparity is typical of [[bony fish]], [[anuran]]s, 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 [[oviduct]]s 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>{{cite book|last1=USA|first1=David O. Norris, Ph.D., Professor Emeritus, Department of Integrative Physiology, University of Colorado at Boulder, Colorado, USA, James A. Carr, Ph.D., faculty director, Joint Admission Medical Program, Department of Biological Sciences, Texas Tech University, Lubbock, Texas|title=Vertebrate endocrinology.|date=2013|isbn=978-0123948151|page=349|publisher=Academic Press |edition=Fifth|url=https://books.google.com/books?id=F_NaW1ZcSSAC&q=uterine+milk+marsupial&pg=PA349|access-date=25 November 2014|url-status=live|archive-url=https://web.archive.org/web/20171101190341/https://books.google.com/books?id=F_NaW1ZcSSAC&pg=PA349&lpg=PA349&dq=uterine+milk+marsupial&source=bl&ots=M_k_K_2IfP&sig=hRHbDN3R6DMto9XgoDyZWtK2pFQ&hl=no&sa=X&ei=-2x0VPWUKuH-ywOCr4K4Bg&ved=0CDAQ6AEwAg#v=onepage&q=uterine%20milk%20marsupial&f=false|archive-date=1 November 2017}}</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 [[placentalia|most mammals]]. Similar structures are found in some sharks and in the lizard ''Pseudomoia pagenstecheri''.<ref>{{cite journal|last1=Hamlett|first1=William C.|title=Evolution and morphogenesis of the placenta in sharks|journal=Journal of Experimental Zoology|date=1989|volume=252|issue=S2|pages=35–52|doi=10.1002/jez.1402520406}}</ref><ref>{{cite journal|last1=Jerez|first1=Adriana|last2=Ramírez-Pinilla|first2=Martha Patricia|title=Morphogenesis of extraembryonic membranes and placentation inMabuya mabouya (Squamata, Scincidae)|journal=Journal of Morphology|date=November 2003|volume=258|issue=2|pages=158–178|doi=10.1002/jmor.10138|pmid=14518010|s2cid=782433}}</ref> In some [[Hylidae|hylid frogs]], the embryo is fed by the mother through specialized [[gill]]s.<ref>{{cite book |title=Vertebrate endocrinology. 1: Morphological considerations |date=1986 |publisher=Academic Press |location=Orlando |isbn=978-0125449014}}</ref>

The term hemotropic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.<ref>{{cite web |url=http://www.etymonline.com/index.php?allowed_in_frame=0&search=histo-%2C+hemo- |title=histo-, hemo- |publisher=Online Etymology Dictionary |access-date=2013-07-27 |url-status=live |archive-url=https://web.archive.org/web/20140514110803/http://www.etymonline.com/index.php?allowed_in_frame=0&search=histo-%2C+hemo- |archive-date=2014-05-14 }}</ref>

==Human use==

===Food===
{{Main|Eggs as food}}
Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia. Popular choices for egg consumption are chicken, duck, roe, and caviar, but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.

====Eggs and Kashrut====
{{See also|Kashrut#Pareve foods|Kosher foods#Eggs}}
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 ({{Bibleverse||Deuteronomy|14:21}}) or stored together. [[egg (food)|Egg]]s 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.<ref>"[https://www.jewishvirtuallibrary.org/jsource/Judaism/kashrut.html Jewish Dietary Laws (Kashrut): Overview of Laws & Regulations]", Jewish Virtual Library. {{webarchive|url=https://web.archive.org/web/20130117080819/http://www.jewishvirtuallibrary.org/jsource/Judaism/kashrut.html |date=2013-01-17 }}.</ref>

===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 [[virus]]es 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===
[[File:Easter Egg Hunt (5623253840).jpg|thumb|upright|Chocolate [[Easter egg]]s 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 in creation myths.<ref>{{Cite web|last=Hall|first=Stephanie|date=2017-04-06|title=The Ancient Art of Decorating Eggs {{!}} Folklife Today|url=https://blogs.loc.gov/folklife/2017/04/decorating-eggs/#:~:text=In%20some%20regions%20solid-color,a%20symbol%20of%20Christ's%20blood|access-date=2021-02-16|website=Library of Congress Blogs}}</ref> [[Egg decorating|Egg decoration]] is a common practice in many cultures worldwide. Christians view [[Easter egg]]s as symbolic of the resurrection of Jesus Christ.<ref>{{Cite news|url=https://www.huffingtonpost.com/2012/04/02/easter-eggs-history-origin-symbolism-tradition_n_1392054.html|title=Easter Eggs: History, Origin, Symbolism And Traditions (PHOTOS)|last=Barooah|first=Jahnabi|date=2012-04-02|work=Huffington Post|access-date=2018-03-31|language=en-AU}}</ref> 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.<ref>{{cite web |url-status=live |url=https://www.youtube.com/watch?v=2Cr4NXT5fd4 |archive-url=https://web.archive.org/web/20160409213049/https://www.youtube.com/watch?v=2Cr4NXT5fd4 |archive-date=2016-04-09 |website=YouTube |title=Dancing Egg in Barcelona Fountain |last1=dznygrl |date=Aug 19, 2009 }}</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.{{Citation needed|date=January 2019}} Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken.<ref>{{Cite news|url=https://www.theguardian.com/world/shortcuts/2015/oct/05/history-egging-political-protest-britain|title=Beyond a yolk: a brief history of egging as a political protest|last=Ramaswamy|first=Chitra|date=2015-10-05|newspaper=The Guardian|language=en|access-date=2018-03-31}}</ref>

==Collecting==
{{Main|Oology}}
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>{{Cite web|url=http://echonewspaper.com.au/collecting-bird-eggs/|title=Collecting bird eggs|website=Echo Newspapers |last1=Mcinnes |first1=Anita |language=en-US|access-date=2018-03-31|date=2017-03-09 |url-status=dead |archive-url=https://web.archive.org/web/20180331173456/http://echonewspaper.com.au/collecting-bird-eggs/ |archive-date=Mar 31, 2018 }}</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>{{Cite web|url=https://www.legislation.gov.uk/ukpga/1954/30/section/1/enacted|title=Protection of Birds Act 1954 - Section 1 |website=legislation.gov.uk|language=en|access-date=2018-03-31}}</ref> On the other hand, ongoing underground trading is becoming a serious issue.<ref>{{Citation|last=Wheeler|first=Timothy|title=Poached|date=2015-03-16|url=https://www.imdb.com/title/tt4283664/ |website=IMDb |access-date=2018-03-31}}</ref>

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>{{Cite web|url=https://australianmuseum.net.au/image/egg-specimens|title=Egg specimens |website=Australian Museum |date=29 April 2009 |first1=    Michael |last1=Sladek |language=en|access-date=2018-03-31 |url-status=dead |archive-url=https://web.archive.org/web/20180331173640/https://australianmuseum.net.au/image/egg-specimens |archive-date=2018-03-31 }}</ref> and the collection in Western Australia Museum has been archived in a gallery.<ref>{{Cite web|url=http://museum.wa.gov.au/explore/galleries/explore-our-egg-collection|title=Explore our Egg Collection |website=Western Australian Museum|language=en|access-date=2018-03-31}}</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>{{Cite news|url=https://www.theatlantic.com/science/archive/2016/03/the-lost-victorian-art-of-egg-collecting/475476/ |url-access=subscription |date= Mar 25, 2016 |title=The Lost Victorian Art of Egg Collecting|last=Golembiewski|first=Kate|work=The Atlantic|access-date=2018-03-31|language=en-US |url-status=live |archive-url=https://web.archive.org/web/20180331173612/https://www.theatlantic.com/science/archive/2016/03/the-lost-victorian-art-of-egg-collecting/475476/ |archive-date= Mar 31, 2018 }}</ref>

==Gallery==
<gallery mode="packed">
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)|Skate]]s and some [[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 [[parasite]]s (mainly [[nematode]]s) from wild [[primate]]s
</gallery>

==See also==
{{Div col}}
*[[List of egg topics]]
*[[Exoskeleton|Animal shell]]
*[[External morphology of Lepidoptera#Egg|Butterfly eggs]]
*[[Egg white]]
*[[Fossil egg]]
*[[Haugh unit]]
*[[Oology]]
*[[Oval]]
*[[Ovary]]
*[[Ovulation]]
*[[Oviparous]]
*[[Trophic egg]]
{{div col end}}

==References==
{{reflist}}

==External links==
{{Wiktionary}}

*{{wikiquote-inline}}
*{{commonscat-inline}}

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{{Birds}}
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[[Category:Eggs| ]]
[[Category:Animal reproductive system]]
[[Category:Aviculture]]
[[Category:Bird breeding]]
[[Category:Oology]]