{{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)}} {{protection padlock|small=yes}} {{Use dmy dates|date=August 2025}} [[File:Oeufs002b.jpg|Eggs of various birds, a turtle, various cartilaginous fish, a cuttlefish and various butterflies and moths. (Click on image for key)|right|thumb|link=File:Adolphe_Millot_oeufs-fixed.jpg#Key]]
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 collecting has been popular in some cultures, although the practice is now banned in many jurisdictions. Chicken eggs are used in the production of vaccines for infectious diseases.
== Eggs of different animal groups == {{Further|Egg cell}}
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 dinosaurs 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 oz).<ref name="Khanna2005"/>{{rp|132}} 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="wikitable" |+ Overview of eggs from various animals ! 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>{{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}} |- | Cartilaginous fish | Macrolecithal eggs with 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}} |- | Bony fish | Macrolecithal eggs, small to medium size, large eggs in the coelacanth<ref name="Romer & Parson"/> | Larval stage, 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> |- | 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. |}
{{anchor|Fish eggs|Amphibian eggs}} === Fish and amphibian eggs === {{redirect|Fish egg|fish eggs as food|Roe}} {{see also|Ichthyoplankton|Spawn (biology)}} {{multiple image | align = left | direction = vertical | 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. 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,<ref name=Schreiber_2023>{{cite book | title=General and Comparative Endocrinology: An Integrative Approach | first=A. M. | last=Schreiber | publisher=CRC Press | year=2023 | isbn=978-1-000-91309-5 | url=https://books.google.com/books?id=AFneEAAAQBAJ&pg=PT1283 | quote=With the exception of mammals, oviparity is the most widely used mode of reproduction among vertebrates, occurring in over 97% of fish, 90% of amphibians, 85% of reptiles, and in 100% of birds. }}</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>{{cite book | chapter=The early life stages of marine fish | first1=Janet T. | last1=Duffy-Anderson | first2=Alison L. | last2=Dreary | first3=Francie | last3=Juanes | first4=Olivier | last4=Le Pape | title=Ecology of Marine Fish | editor1-first=Henrique | editor1-last=Cabral | editor2-first=Mario | editor2-last=LePage | editor3-first=Jeremy | editor3-last=Lobry | editor4-first=Olivier | editor4-last=Le Pape | publisher=Elsevier | year=2024 | isbn=978-0-323-99037-0 | pages=47–50 | chapter-url=https://books.google.com/books?id=vGYGEQAAQBAJ&pg=PA47 }}</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>{{cite book | title=Meat Eaters: Raptors, Sharks, and Crocodiles | series=The Britannica Guide to Predators and Prey | editor-first=John P. | editor-last=Rafferty | publisher=The Rosen Publishing Group, Inc | year=2011 | isbn=978-1-61530-342-7 | page=108 | url=https://books.google.com/books?id=OLA0ZGJx0ccC&pg=PA108 }}</ref><ref>{{cite book | chapter=Reproductive Strategies of Fish | first=Robert A. | last=Patzner | title=Fish Reproduction | editor1-first=Maria J. | editor1-last=Rocha | editor2-first=Augustine | editor2-last=Arukwe | editor3-first=B. G. | editor3-last=Kapoor | publisher=CRC Press | year=2008 | page=324 | isbn=978-1-4398-4239-3 | chapter-url=https://books.google.com/books?id=qdX9DwAAQBAJ&pg=PA324 }}</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>{{cite book | title=Red Sea Sharks | series=In depth divers' guide | first=Jeremy | last=Stafford-Deitsch | publisher=Trident Press Ltd | year=1999 | isbn=978-1-900724-36-4 | url=https://books.google.com/books?id=xu9CFiC3fwgC&pg=PA32 }}</ref>
[[File:RanaArvalisMatingPlusSpawn.jpg|thumb|Moor frog eggs utilize glycans to form a hydrophilic jelly coat that protects the egg<ref>{{Cite journal | last1=Shu | first1=Longfei | last2=Suter | first2=Marc J.-F. | last3=Laurila | first3=Anssi | last4=Räsänen | first4=Katja | date=November 2015 | title=Mechanistic basis of adaptive maternal effects: egg jelly water balance mediates embryonic adaptation to acidity in Rana arvalis | journal=Oecologia | volume=179 | issue=3 | pages=617–628 | doi=10.1007/s00442-015-3332-4 | pmid=25983113 | bibcode=2015Oecol.179..617S | hdl=20.500.11850/101187 | s2cid=253976911 | issn=1432-1939 | hdl-access=free }}</ref>]]
The eggs of fish and amphibians (anamniotes) are jellylike.<ref name=Bonnan_2016>{{cite book | title=The Bare Bones: An Unconventional Evolutionary History of the Skeleton | series=Life of the Past | first=Matthew F. | last=Bonnan | publisher=Indiana University Press | year=2016 | isbn=978-0-253-01841-0 | pages=228–229 | url=https://books.google.com/books?id=sENNCwAAQBAJ&pg=PA228 }}</ref> Cartilaginous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development.<ref>{{cite book | title=The Central Nervous System of Cartilaginous Fishes: Structure and Functional Correlations | first1=W. J. A. J. | last1=Smeets | first2=R. | last2=Nieuwenhuys | first3=B. L. | last3=Roberts | publisher=Springer Science & Business Media | year=2012 | isbn=978-3-642-68923-9 | url=https://books.google.com/books?id=R5bzCAAAQBAJ&pg=PA4 }}</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>{{cite book | title=Biology and Ecology of Fishes | first1=James S. | last1=Diana | first2=Tomas O. | last2=Höök | edition=3rd | publisher=John Wiley & Sons | year=2023 | isbn=978-1-119-50574-7 | pages=297–298 | url=https://books.google.com/books?id=PFrEEAAAQBAJ&pg=PA297 }}</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>{{cite book | title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution | series=Mariner Book | first=Richard | last=Dawkins | author-link=Richard Dawkins | publisher=Houghton Mifflin Harcourt | year=2004 | isbn=978-0-618-61916-0 | url=https://books.google.com/books?id=rR9XPnaqvCMC&pg=PA294 }}</ref>
=== Amniote eggs and embryos === [[File:Snapping turtle eggs md.jpg|thumb|right|Turtle eggs in a nest dug by a female common snapping turtle (''Chelydra serpentina'')]] Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or embryos, including an amniotic membrane.<ref>{{cite book | chapter=Evolution of the Amniote egg | first1=Mary J. | last1=Packard | first2=Roger S. | last2=Seymour | title=Amniote Origins: Completing the Transition to Land | editor1-first=Stuart | editor1-last=Sumida | editor2-first=Karen L. M. | editor2-last=Martin | publisher=Elsevier | year=1997 | isbn=978-0-08-052709-3 | pages=266–274 | chapter-url=https://books.google.com/books?id=9f7rafLJbWUC&pg=PA266 }}</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>{{cite book | title=Reproduction in Mammals: The Female Perspective | first1=Virginia | last1=Hayssen | first2=Teri J. | last2=Orr | publisher=JHU Press | year=2017 | isbn=978-1-4214-2316-6 | pages=10–15 | url=https://books.google.com/books?id=vwE2DwAAQBAJ&pg=PA10 }}</ref> [[File:Black Caiman, São Carlos, Porto Velho - RO, Brasil imported from iNaturalist photo 342198284.jpg|thumb|Black caiman egg showing the different layers]] 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>{{cite book | chapter=Perinatology | first=Dougls R. | last=Mader | title=Reptile Medicine and Surgery | editor1-first=Stephen J. | editor1-last=Divers | editor2-first=Douglas R. | editor2-last=Mader | edition=2nd | publisher=Elsevier Health Sciences | year=2005 | isbn=978-1-4160-6477-0 | page=366 | chapter-url=https://books.google.com/books?id=7Ai4BKhi0VUC&pg=PA366 }}</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>{{cite book | chapter=Overview of Biology, Anatomy, and Histology of Reptiles | first1=Elliott R. | last1=Jacobson | first2=Harvey B. | last2=Lilleywhite | first3=Daniel G. | last3=Blackburn | title=Diseases and Pathology of Reptiles | editor1-first=Elliott | editor1-last=Jacobson | editor2-first=Michael | editor2-last=Garner | publisher=CRC Press | year=2021 | edition=2nd | volume=1 | isbn=978-0-429-63295-2 | chapter-url=https://books.google.com/books?id=BoM_EAAAQBAJ&pg=PA29 }}</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>{{cite journal | title=Understanding the evolution of viviparity using intraspecific variation in reproductive mode and transitional forms of pregnancy | first1=Camilla M. | last1=Whittington | first2=James U. | last2=Van Dyke | first3=Stephanie Q. T. | last3=Liang | first4=Scott V. | last4=Edwards | first5=Richard | last5=Shine | first6=Michael B. | last6=Thompson | first7=Catherine E. | last7=Grueber | journal=Biological Reviews | volume=97 | issue=3 | date=June 2022 | pages=1179–1192 | doi=10.1111/brv.12836 | pmid=35098647 | pmc=9064913 }}</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>{{cite journal | title=The first dinosaur egg was soft | first1=Mark A. | last1=Norell | first2=Jasmina | last2=Wiemann | first3=Matteo | last3=Fabbri | first4=Congyu | last4=Yu | first5=Claudia A. | last5=Marsicano | first6=Anita | last6=Moore-Nall | first7=David J. | last7=Varricchio | first8=Diego | last8=Pol | first9=Darla K. | last9=Zelenitsky | journal=Nature | volume=583 | pages=406–410 | year=2020 | issue=7816 | doi=10.1038/s41586-020-2412-8 | pmid=32555457 | bibcode=2020Natur.583..406N | hdl=11336/109490 | hdl-access=free }}</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 extant order of egg-laying mammal.<ref>{{cite journal | title=Review of the monotreme fossil record and comparison of palaeontological and molecular data | first=A. M. | last=Musser | journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology | volume=136 | issue=4 | date=December 2003 | pages=927–942 | doi=10.1016/S1095-6433(03)00275-7 | pmid=14667856 }}</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 === [[File:Chicken egg diagram.svg|thumb|Diagram of a fertilized chicken egg in its ninth day]] thumb|Six commercial chicken eggs {{main|Bird egg}}
Bird eggs are laid by females and incubated for a time that varies according to the species;<ref>{{cite book | title=The Maryland Master Naturalist's Handbook | first=Joy Shindler | last=Rafey | editor-first=McKay | editor-last=Jenkins | publisher=JHU Press | year=2025 | isbn=978-1-4214-5159-6 | url=https://books.google.com/books?id=_48UEQAAQBAJ&pg=PT148 }}</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>{{cite journal | title=The double-yolked egg: from the 'miracle of packaging' to nature's 'mistake' | first1=Attila | last1=Salamon | first2=John P. | last2=Kent | journal=World's Poultry Science Journal | volume=76 | issue=1 | pages=18–33 | date=19 March 2020 | doi=10.1080/00439339.2020.1729671 }}</ref> One case of twin geese has been observed to hatch from an elongated egg.<ref>{{cite journal | title=Elongated shape and unusual eggshell microstructure enable first confirmed hatching of avian twins | first1=Krzysztof | last1=Damaziak | first2=Agata | last2=Marzec | first3=Wojciech | last3=Wójcik | first4=Beata | last4=Horecka | first5=Mateusz | last5=Osiadacz | first6=Julia | last6=Riedel | first7=Paweł | last7=Pstrokoński | first8=Sebastian | last8=Mielnicki | journal=Biology of Reproduction | volume=113 | issue=1 | date=July 2025 | pages=49–60 | doi=10.1093/biolre/ioaf100 | pmid=40302037 }}</ref> Average clutch sizes range from one (as in condors<ref>{{cite journal | title=Life History of the Andean Condor in Ecuador | first1=Juan Sebastián | last1=Restrepo-Cardona | first2=Fabricio | last2=Narváez | first3=Sebastián | last3=Kohn | first4=Rubén | last4=Pineida | first5=Félix Hernán | last5=Vargas | journal=Tropical Conservation Science | date=2024 | volume=17 | pages=1–8 | article-number=19400829241238005 | doi=10.1177/19400829241238005 | doi-access=free }}</ref>) to about 17–24 (the grey partridge<ref>{{cite journal | title=Breeding dispersal and demography of wild and hand-reared grey partridges Perdix perdix in Finland | first1=Ahti | last1=Putaala | first2=Raimo | last2=Hissa | journal=Wildlife Biology | volume=4 | issue=3 | date=September 1998 | pages=137–145 | doi=10.2981/wlb.1998.016 | bibcode=1998WildB...4..137P }}</ref>). It is rare for a bird to lay eggs when not fertilized,<ref>{{cite book | title=Egg: A Dozen Ovatures | first=Lizzie | last=Stark | publisher=W. W. Norton & Company | year=2023 | isbn=978-0-393-53151-0 | url=https://books.google.com/books?id=tHZwEAAAQBAJ&pg=PT8 }}</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>{{cite book | title=Advances in Agriculture Research and Application | edition=2011 | editor-first=Q. Ashton | editor-last=Acton | publisher=ScholarlyEditions | year=2012 | isbn=978-1-4649-2119-3 | page=528 | url=https://books.google.com/books?id=JBLlIilam5MC&pg=PA528 }}</ref> which are sometimes called wind-eggs.<ref>{{cite book | title=Ancient and Modern Approaches to the Problem of Relativism: A Study of Husserl, Locke, and Plato | series=Recovering Political Philosophy | first=Matthew K. | last=Davis | publisher=Springer Nature | year=2023 | isbn=978-3-031-22304-4 | url=https://books.google.com/books?id=ncu_EAAAQBAJ&pg=PA66 }}</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>{{cite journal | title=The evolution of biomineralization in metazoans | last1=Debiais-Thibaud | first1=M. | last2=Marin | first2=F. | last3=Marcellini | first3=S. | journal=Frontiers in Genetics | volume=13 | year=2023 | article-number=1092695 | doi=10.3389/fgene.2022.1092695 | pmid=36685829 | pmc=9848429 | doi-access=free }}</ref> They vary in thickness from paper thin up to {{Val|2.7|u=mm}} in ostriches, and typically form {{Val|11|-|15|u=%}} of the egg's weight.<ref name=Gill_2007/> Bird eggshells are diverse in appearance and structure.<ref name=Gill_2007>{{cite book | title=Ornithology | first=Frank B. | last=Gill | publisher=Macmillan | year=2007 | isbn=978-0-7167-4983-7 | url=https://books.google.com/books?id=zM0tG5ApO0UC&pg=PA421 }}</ref> For example: *cormorant eggs are rough and chalky<ref>{{cite book | title=Bird-life: A Guide to the Study of Our Common Birds | first=Frank Michler | last=Chapman | others=illustrated by Ernest Thompson Seton | date=1919 | publisher=D. Appleton and Company=1919 | url=https://books.google.com/books?id=urkpAQAAIAAJ&pg=PA71 }}</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>{{cite book | title=Essentials of Avian Medicine and Surgery | editor-first=Brian H. | editor-last=Coles | publisher=John Wiley & Sons | year=2008 | isbn=978-0-470-69156-4 | page=203 | url=https://books.google.com/books?id=Y__okjurvl8C&pg=PA203 }}</ref> The domestic hen's egg has around 7000 pores.<ref>{{Cite web | title=The Parts of the Egg | 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 | archivedate=23 November 2016 | work=4-H Virtual Farm }}</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>{{cite journal | last1=Portugal | first1=J. P. | last2=Bowen | first2=J. | last3=Riehl | first3=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]]
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>{{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 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:41718764 | access-date=2025-08-05 | 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 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>{{cite journal | first=Yutaka | last=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>
==== Colors ==== [[File:Uria aalge MHNT Box Rouzic.jpg|thumb|Guillemot eggs]] The default color of avian eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce colored eggs. The color comes from pigments deposited on top of the calcium carbonate base; biliverdin and its zinc chelate, and bilirubin, give a green or blue ground color, while protoporphyrin IX produces reds and browns as a ground color 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 |publisher=Australian Broadcasting Corporation |access-date=21 January 2023}}</ref><ref>{{cite journal | title=To Prevent Oxidative Stress, What about Protoporphyrin IX, Biliverdin, and Bilirubin? | first1=Ana | last1=Martínez | first2=Isabel | last2=López-Rull | first3=Juan A. | last3=Fargallo | journal=Antioxidants | date=23 August 2023 | volume=12 | issue=9 | page=1662 | doi=10.3390/antiox12091662 | doi-access=free | pmid=37759965 | pmc=10525153 }}</ref><ref name=Kilner_2006/> Shell colors 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>{{cite book | title=Birdpedia: A Brief Compendium of Avian Lore | series=Pedia Books | first=Christopher W. | last=Leahy | others=Illustrated by Abby McBride | publisher=Princeton University Press | year=2021 | isbn=978-0-691-21823-6 | page=87 | url=https://books.google.com/books?id=M80QEAAAQBAJ&pg=PA87 }}</ref>
Non-passerines typically have white eggs,<ref name=Attard_et_al_2023>{{cite journal | title=Surface texture heterogeneity in maculated bird eggshells | first1=Marie R. G. | last1=Attard | first2=James | last2=Bowen | first3=Steven J. | last3=Portugal | journal=Journal of the Royal Society Interface | date=12 July 2023 | volume=20 | issue=204 | article-number=20230293 | doi=10.1098/rsif.2023.0293 | pmid=37434502 | pmc=10336372 }}</ref> except in some ground-nesting groups such as the Charadriiformes,<ref name=Attard_et_al_2023/> sandgrouse,<ref>{{cite journal | last=Maclean | first=C. L. | year=1985 | title=Sandgrouse: models of adaptive compromise | journal=South African Journal of Wildlife Research | volume=15 | issue=1 | pages=1–6 | url=https://journals.co.za/doi/pdf/10.10520/AJA03794369_3539 | access-date=2025-08-05 }}</ref> and common terns,<ref>{{cite journal | title=All around the egg: consistency of spottiness and colouration across an avian eggshell | last1=Minias | first1=Piotr | last2=Gómez | first2=Jesús | last3=Janiszewski | first3=Tomasz | journal=Journal of Ornithology | volume=165 | issue=3 | pages=703–711 | date=July 2024 | doi=10.1007/s10336-024-02162-3 | bibcode=2024JOrni.165..703M }}</ref> where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg.<ref>{{cite book | title=The Nesting Season: Cuckoos, Cuckolds, and the Invention of Monogamy | first=Bernd | last=Heinrich | publisher=Harvard University Press | year=2010 | isbn=978-0-674-04877-5 | page=257 | url=https://books.google.com/books?id=pYbAknzzzz8C&pg=PA257 }}</ref> 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.<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|bibcode=2005EcolL...8.1105G }}</ref> Later eggs in a clutch are more spotted than early ones as the female's pigment glands become depleted.<ref name=Kilner_2006>{{cite journal | title=The evolution of egg colour and patterning in birds | first=R. M. | last=Kilner | journal=Biological Reviews | year=2006 | volume=81 | issue=3 | pages=383–406 | doi=10.1017/S1464793106007044 | pmid=16740199 | quote=its colouring may be the non-adaptive consequence of pigment glands becoming depleted, or emptying themselves entirely with the completion of the clutch }}</ref>
Within the common cuckoo lineage, the color 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 color in other species is most likely inherited from both parents.<ref>{{cite journal | title=Ancient origin and maternal inheritance of blue cuckoo eggs | last1=Fossøy | first1=Frode | last2=Sorenson | first2=Michael D. | last3=Liang | first3=Wei | last4=Ekrem | first4=Torbjørn | last5=Moksnes | first5=Arne | last6=Møller | first6=Anders P. | last7=Rutila | first7=Jarkko | last8=Røskaft | first8=Eivin | last9=Takasu | first9=Fugo | last10=Yang | first10=Canchao | last11=Stokke | first11=Bård G. | journal=Nature Communications | volume=7 | at=id. 10272 | date=January 2016 | article-number=10272 | doi=10.1038/ncomms10272 | pmid=26754355 | pmc=4729921 | bibcode=2016NatCo...710272F | hdl=11250/2427283 | hdl-access=free }}</ref> For chickens, egg color appears determined from the hen's genome, diet, and stress factors like disease.<ref>{{Cite web | last=Schattenberg | first=Paul | date=April 2025 | title=Why are eggs different colors? | url=https://agrilifetoday.tamu.edu/2025/04/01/why-are-eggs-different-colors/ | access-date=2025-08-08 | website=AgriLife Today | language=en-US }}</ref> With American robins, there is some evidence that the brightness of the egg coloration may influence male parental care of the nestlings.<ref name=English_Montgomerie_2011>{{cite journal | title=Robin's egg blue: does egg color influence male parental care? | first1=Philina A. | last1=English | first2=Robert | last2=Montgomerie | journal=Behavioral Ecology and Sociobiology | volume=65 | issue=5 | date=May 2011 | pages=1029–1036 | doi=10.1007/s00265-010-1107-9 | jstor=41414064 | bibcode=2011BEcoS..65.1029E }}</ref>
Evolutionary factors can drive egg coloration, such as predation selecting for cryptic coloration, or colorful eggs possibly being used to coerce males into providing additional care during incubation – the blackmail hypothesis.<ref>{{cite journal | title=Parents, predators, parasites, and the evolution of eggshell colour in open nesting birds | first1=Daniel | last1=Hanley | first2=Phillip | last2=Cassey | first3=Stéphanie M. | last3=Doucet | journal=Evolutionary Ecology | volume=27 | pages=593–617 | year=2013 | issue=3 | doi=10.1007/s10682-012-9619-6 | bibcode=2013EvEco..27..593H }}</ref> For avian species that play host to brood parasite eggs, selection pressure drives the host species to evolve distinctive egg colorations 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 coloration evolutionary arms race between the host and parasite.<ref name=Stoddard_et_al_2017>{{cite journal | title=Colour, vision and coevolution in avian brood parasitism | first1=Mary Caswell | last1=Stoddard | first2=Mark E. | last2=Hauber | journal= Philosophical Transactions of the Royal Society B: Biological Sciences| date=5 July 2017 | volume=372 | issue=1724 | id=20160339 | doi=10.1098/rstb.2016.0339 | pmid=28533456 | pmc=5444060 }}</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>{{cite journal | title=The chemical basis of a signal of individual identity: shell pigment concentrations track the unique appearance of Common Murre eggs | first1=Mark E. | last1=Hauber | first2=Alexander L. | last2=Bond | first3=Amy-Lee | last3=Kouwenberg | first4=Gregory J. | last4=Robertson | first5=Erpur S. | last5=Hansen | first6=Mande | last6=Holford | first7=Miri | last7=Dainson | first8=Alec | last8=Luro | first9=James | last9=Dale | journal=Journal of the Royal Society Interface | date=April 2019 | volume=16 | issue=153 | doi=10.1098/rsif.2019.0115 | pmid=30966949 | pmc=6505551 }}</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>{{cite journal | title=Black Oystercatcher Habitat Selection, Reproductive Success, and Their Relationship with Glaucous-Winged Gulls | first1=K. | last1=Vermeer | first2=K. H. | last2=Morgan | first3=G. E. J. | last3=Smith | journal=Colonial Waterbirds | volume=15 | issue=1 | year=1992 | pages=14–23 | doi=10.2307/1521350 | jstor=1521350 }}</ref><ref>{{cite book | title=Kodiak Island Wildlife: Biology and Behavior of the wild animals of Alaska's Emerald Isle | first=Robin | last=Barefield | publisher=Publication Consultants | year=2021 | isbn=978-1-63747-010-7 | url=https://books.google.com/books?id=3cJxEQAAQBAJ&pg=PT295 }}</ref><ref>{{cite journal | title=Black Oystercatcher (''Haematopus bachmani'') Conservation Action Plan | date=February 2010 | version=1.1 | first1=David F. | last1=Tessler | first2=James A. | last2=Johnson | first3=Brad A. | last3=Andres | first4=Sue | last4=Thomas | first5=Richard | last5=Lanctot | journal=International Wader Studies | volume=20 | issue=83 | page=15 | url=https://avibirds.com/wp-content/uploads/pdf/black-oystercatcher2010.pdf | access-date=2025-08-06 }}</ref> The stoat (''Mustela erminea'') and long-tailed weasel (''M. frenata'') steal ducks' eggs.<ref>{{cite journal | title=Predation by ermine and long-tailed weasels on duck eggs | last=Fleskes | first=J. P. | journal=Journal of the Iowa Academy of Science | volume=95 | pages=14–17 | year=1988 | bibcode=1988JIaAS..95...14F | url=https://scholarworks.uni.edu/jias/vol95/iss1/6/ | access-date=2025-08-07 }}</ref> Snakes of the genera ''Dasypeltis'' and ''Elachistodon'' specialize in eating eggs.<ref>{{cite journal | title=Snake predators of bird eggs: a review and bibliography | last1=Barends | first1=J. M. | first2=B. | last2=Maritz | year=2022 | journal=Journal of Field Ornithology | volume=93 | issue=2 | page=1 | doi=10.5751/JFO-00088-930201 | doi-access=free }}</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>{{cite journal | title=Egg Destruction and Egg Removal by Avian Brood Parasites: Adaptiveness and Consequences | first=Brian D. | last=Peer | journal=The Auk | volume=123 | issue=1 | date=January 2006 | pages=16–22 | publisher=Oxford University Press | doi=10.1093/auk/123.1.16 | jstor=4090624 }}</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>{{cite book | last1=Colbert | first1=H. E. | last2=Morales | first2=M. | year=1991 | title=Evolution of the Vertebrates – A History of Backboned Animals Through Time | edition=4th | publisher=John Wiley & Sons inc | location=New York City | isbn=0-471-85074-8 }}</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>{{cite book | chapter=Toxicity of the pregnant female reproductive system | last1=Basavarajappa | first1=M. | last2=Peretz | first2=J. | last3=Paulose | first3=T. | last4=Gupta | first4=R. | last5=Ziv-Gal | first5=A. | last6=Flaws | first6=J. A. | title=Reproductive Toxicology | series=Target Organ Toxicology Series | editor1-first=Robert W. | editor1-last=Kapp | editor2-first=Rochelle W. | editor2-last=Tyl | edition=3rd | publisher=CRC Press | year=2016 | isbn=978-1-4200-7344-7 | page=295 | chapter-url=https://books.google.com/books?id=YQTLBQAAQBAJ&pg=PA295 }}</ref> 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 invertebrates, including insects, spiders,<ref>{{cite book | title=Invertebrate Histology | editor-first=Elise E. B. | editor-last=LaDouceur | publisher=John Wiley & Sons | year=2021 | isbn=978-1-119-50765-9 | page=222 | url=https://books.google.com/books?id=1-gSEAAAQBAJ&pg=PA222 }}</ref> mollusks,<ref>{{cite book | title=Sea Fishes Of The Mediterranean Including Marine Invertebrates | first=Lawson | last=Wood | publisher=Bloomsbury Publishing | year=2015 | isbn=978-1-4729-2177-2 | page=58 | url=https://books.google.com/books?id=Zm1YCgAAQBAJ&pg=PA58 }}</ref> and crustaceans.<ref>{{cite book | last=Susanto | first=G. N. | year=2021 | chapter=Crustacea: The increasing economic importance of crustaceans to humans | title=Arthropods-Are They Beneficial for Mankind? | editor-first=Ramón E. R. | editor-last=Ranz | publisher=Books on Demand | isbn=978-1-78984-165-7 | chapter-url=https://books.google.com/books?id=abZaEAAAQBAJ&pg=PA171 }}</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 yolk 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>{{cite book | title=An Introduction to the Invertebrates | first=Janet | last=Moore | edition=2 | publisher=Cambridge University Press | year=2006 | isbn=978-1-139-45847-4 | pages=247–248 | url=https://books.google.com/books?id=bZw-ntFxp-YC&pg=PA248 }}</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>{{cite book | title=Father Nature: The Science of Paternal Potential | first=James K. | last=Rilling | publisher=MIT Press | year=2024 | isbn=978-0-262-04893-4 | pages=48–50 | url=https://books.google.com/books?id=JzDuEAAAQBAJ&pg=PA48 }}</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>{{cite encyclopedia | title=Male Reproduction | first=Bernard | last=Jégou | series=Encyclopedia of Reproduction | volume=1 | editor-first=Michael K. | editor-last=Skinner | edition=2 | publisher=Academic Press | year=2018 | isbn=978-0-12-815145-7 | page=36 | url=https://books.google.com/books?id=m4RlDwAAQBAJ&pg=RA5-PA36 }}</ref> Unisexual reproduction is uncommon in vertebrates, but has been observed in some fish, reptile, and amphibian taxa.<ref>{{cite book | title=Comparative Vertebrate Reproduction | first=Julian | last=Lombardi | publisher=Springer Science & Business Media | year=2012 | isbn=978-1-4615-4937-6 | pages=51–54 | url=https://books.google.com/books?id=tXvjBwAAQBAJ&pg=PA51 }}</ref>
==Evolution and structure== All sexually reproducing life, including both plants and animals, produces gametes.<ref>{{Cite web | date=18 December 2021 | title=11.3: Sexual Reproduction | url=https://bio.libretexts.org/Courses/Reedley_College/Biology_for_Science_Majors_I/11%3A_Meiosis_and_Sexual_Reproduction/11.03%3A_Sexual_Reproduction | access-date=2025-08-08 | website=Biology LibreTexts | language=en }}</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>{{cite book | title=Animal Behaviour: An Evolutionary Perspective | first=Peter M. | last=Kappeler | publisher=Springer Nature | year=2022 | isbn=978-3-030-82879-0 | pages=153–157 | url=https://books.google.com/books?id=z2ZbEAAAQBAJ&pg=PA153 }}</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>{{cite book | title=Development and Reproduction in Humans and Animal Model Species | first1=Werner A. | last1=Mueller | first2=Monika | last2=Hassel | first3=Maura | last3=Grealy | publisher=Springer | year=2015 | isbn=978-3-662-43784-1 | url=https://books.google.com/books?id=I1wMBgAAQBAJ&pg=PA20 }}</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>{{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 | year=2011 | volume=316 | issue=7 | pages=467–483 | doi=10.1002/jez.b.21417 | pmid=21557469 }}</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>{{cite journal | last1=Byrne | first1=B. M. | last2=Gruber | first2=M. A. B. G. | last3=Ab | first3=G. | year=1989 | title=The evolution of egg yolk proteins | journal=Progress in Biophysics and Molecular Biology | volume=53 | issue=1 | pages=33–69 | doi=10.1016/0079-6107(89)90005-9 | pmid=2682782 }}</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>{{cite book | chapter=Morphology and Evolution of the Egg of Oviparous Amniotes | title=Amniote Origins: Completing the Transition to Land | editor1-first=Stuart | editor1-last=Sumida | editor2-first=Karen L. M. | editor2-last=Martin | publisher=Elsevier | year=1997 | isbn=978-0-08-052709-3 | page=309 | chapter-url=https://books.google.com/books?id=9f7rafLJbWUC&pg=PA309 }}</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>{{cite book | chapter=Ovalbumin and Gene-related Proteins | last1=Lechevalier | first1=V. | last2=Croguennec | first2=T. | last3=Nau | first3=F. | last4=Guérin-Dubiard | first4=C. | year=2007 | title=Bioactive egg compounds | pages=51–60 | location=Berlin, Heidelberg | publisher=Springer Berlin Heidelberg | isbn=978-3-540-37885-3 | chapter-url=https://books.google.com/books?id=hxVZmaaPaK8C&pg=PA51 }}</ref> Ovomucoid, which makes up 11% of the albumen, is the primary egg allergen.<ref>{{cite journal | last1=Ahmed | first1=T. | last2=Ayaz | first2=H. | last3=Nisar | first3=D. | last4=Moin | first4=A. | year=2024 | title=Ovomucoid (the most important Egg white allergen) as a cause of severe Egg allergy: A review | journal=Sanamed | volume=19 | issue=3 | pages=325–332 | doi=10.5937/sanamed0-51327 | doi-access=free }}</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>{{cite book | title=Clinical Anatomy and Physiology for Veterinary Technicians | first1=Thomas P. | last1=Colville | first2=Joanna M. | last2=Bassert | edition=3rd | publisher=Elsevier Health Sciences | year=2015 | isbn=978-0-323-35620-6 | page=558 | url=https://books.google.com/books?id=9DMwBwAAQBAJ&pg=PA558 }}</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>{{cite book | title=Understanding Reproduction | series=Understanding Life | first1=Giuseppe | last1=Fusco | first2=Alessandro | last2=Minelli | publisher=Cambridge University Press | year=2023 | isbn=978-1-009-22593-9 | page=153 | url=https://books.google.com/books?id=qt_LEAAAQBAJ&pg=PA153 }}</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">{{cite book | author1-link=Alfred Romer | last1=Romer | first1=A. S. | last2=Parsons | first2=T. S. | year=1985 | title=The Vertebrate Body | edition=6th | publisher=Saunders | location=Philadelphia }}</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>{{cite book | title=Human Life Before Birth | first=Frank | last=Dye | publisher=CRC Press | year=2000 | isbn=978-90-5702-608-9 | pages=60–61 | url=https://books.google.com/books?id=F8lBlJ1wJWEC&pg=PA60 }}</ref>
==== Microlecithal ==== [[File:Toxocara embryonated eggs.jpg|thumb|right|Microlecithal eggs from the 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 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>{{cite journal | last1=Reitzel | first1=A. M. | last2=Sullivan | first2=J. C. | last3=Finnery | first3=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 flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods.<ref name="Barns">{{cite book | last=Barns | first=R. D. | year=1968 | title=Invertebrate Zoology | publisher=W. B. Saunders Company | location=Philadelphia }}</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>{{cite book | title=Mammalian Development | first=Manju | last=Yadav | publisher=Discovery Publishing House | year=2008 | isbn=978-81-8356-299-7 | page=117 | url=https://books.google.com/books?id=GNwMf97HIe8C&pg=PA117 }}</ref>
====Mesolecithal==== [[File:Frogspawn closeup.jpg|thumb|right|Frogspawn 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>{{cite book | last1=Hildebrand | first1=M. | last2=Gonslow | first2=G. | year=2001 | title=Analysis of Vertebrate Structure | edition=5th | publisher=John Wiley & Sons, Inc | location=New York City | isbn=978-0-471-29505-1 }}</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>{{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 lampreys, coelacanth and the salamanders.<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: Cephalopods and vertebrates.<ref name="Romer & Parson"/><ref>{{cite conference | editor-last1=Nixon | editor-first1=M. | editor2-last=Messenger | editor2-first=J. B. | year=1977 | title=The Biology of Cephalopods | work=Symposium of the Zoological Society of London | pages=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">{{cite book | last=Stewart | first=J. R. | year=1997 | chapter=Morphology and evolution of the egg of oviparous amniotes | editor1-first=S. | editor1-last=Sumida | editor2-first=K. | editor2-last=Martin | title=Amniote Origins-Completing the Transition to Land | issue=1 | pages=291–326 | location=London | publisher=Academic Press | isbn=978-0-08-052709-3 | chapter-url=https://books.google.com/books?id=9f7rafLJbWUC&pg=PA291 }}</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 {{cvt|9|cm}} in diameter, and the young go through full development while in the uterus, living on the copious yolk.<ref>{{cite journal | last1=Fricke | first1=H. W. | last2=Frahm | first2=J. | date=October 1992 | title=Evidence for lecithotrophic viviparity in the living coelacanth | journal=Naturwissenschaften | volume=79 | issue=10 | pages=476–479 | doi=10.1007/BF01139204 | bibcode=1992NW.....79..476F | hdl=11858/00-001M-0000-0013-08C9-D | hdl-access=free }}</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>{{cite journal | first=Lodé | last=T. | year=2012 | title=Oviparity or viviparity? That is the question… | journal=Reproductive Biology | volume=12 | issue=3 | pages=259–264 | doi=10.1016/j.repbio.2012.09.001 | pmid=23153695 }}</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>{{cite book | first1=David O. | last1=Norris | first2=James A. | last2=Carr | title=Vertebrate endocrinology | date=2013 | isbn=978-0-12-394815-1 | page=349 | publisher=Academic Press | edition=Fifth | url=https://books.google.com/books?id=F_NaW1ZcSSAC&pg=PA349 }}</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>{{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 | bibcode=1989JEZ...252S..35H }}</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 | bibcode=2003JMorp.258..158J | s2cid=782433 }}</ref> In some hylid frogs, the embryo is fed by the mother through specialized gills.<ref>{{cite book | chapter=Morphological considerations | first=Ivor M. D. | last=Jackson | title=Vertebrate Endocrinology: Fundamentals and Biomedical Considerations | editor1-first=Peter K. T. | editor1-last=Pang | editor2-first=Martin P. | editor2-last=Schreibman | publisher=Elsevier | year=2012 | isbn=978-0-323-14759-0 | chapter-url=https://books.google.com/books?id=9lPSuRtIHjYC&pg=PA1 }}</ref>
The term hemotrophic 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}} {{Cookbook|Egg}} Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia.<ref>{{cite book | title=Eggs in Cookery | series=Proceedings of the Oxford Symposium of Food and Cookery | editor-first=Richard | editor-last=Hosking | publisher=Oxford Symposium | year=2007 | isbn=978-1-903018-54-5 | pages=100–101 | url=https://books.google.com/books?id=cfP6jHmSLnMC&pg=PT100 }}</ref> Popular choices for egg consumption are chicken, duck, roe, and caviar,<ref>{{cite book | title=Microbial Food Safety: A Food Systems Approach | first1=Charlene | last1=Wolf-Hall | first2=William | last2=Nganje | publisher=CABI | year=2017 | isbn=978-1-78064-480-6 | page=5 | url=https://books.google.com/books?id=raJ2DgAAQBAJ&pg=PA5 }}</ref> but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.<ref>{{cite book | chapter=Chicken Eggs | first=William J. | last=Stadelman | title=The Cambridge World History of Food | volume=1 | series=Cambridge histories online | editor1-first=Kenneth F. | editor1-last=Kiple | editor2-first=Kriemhild Coneè | editor2-last=Ornelas | publisher=Cambridge University Press | year=2000 | isbn=978-0-521-40214-9 | pages=499–507 | chapter-url=https://books.google.com/books?id=RSSkDNzKQacC&pg=PA499 }}</ref>
====Eggs and Kashrut==== {{See also|Kashrut#Pareve foods|Kosher foods#Eggs}} 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>{{cite web | url=https://www.jewishvirtuallibrary.org/jsource/Judaism/kashrut.html | date=17 January 2013 | title=Jewish Dietary Laws (Kashrut): Overview of Laws & Regulations | publisher=Jewish Virtual Library | archive-url=https://web.archive.org/web/20130117080819/http://www.jewishvirtuallibrary.org/jsource/Judaism/kashrut.html | archive-date=17 January 2013 }}</ref>
===Vaccine manufacture=== thumb|Preparation of measles vaccine at the Tirana (Albania) Institute of Hygiene and Epidemiology. Two technicians, wearing surgical gowns, are making small openings in eggs. (Photo by WHO) Many vaccines for infectious diseases are produced in fertile chicken eggs.<ref>{{Cite web | date=30 September 2024 | title=How Influenza (Flu) Vaccines Are Made | url=https://www.cdc.gov/flu/vaccine-process/index.html | access-date=2025-08-08 | publisher=CDC | language=en-us }}</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>{{cite journal | title=The Cultivation of Vaccine and Other Viruses in the Chorio-Allantoic Membrane of Chick Embryos | first1=E. W. | last1=Goodpasture | first2=Alice M. | last2=Woodruff | first3=G. J. | last3=Buddingh | journal=Science | date=9 Oct 1931 | volume=74 | issue=1919 | pages=371–372 | doi=10.1126/science.74.1919.371 | pmid=17810781 | bibcode=1931Sci....74..371G }}</ref> This enabled the development of vaccines against influenza, chicken pox, smallpox, yellow fever, typhus, Rocky mountain spotted fever and other diseases.<ref>{{cite book | title=A Shot in the Arm: How Science, Engineering, and Supply Chains Converged to Vaccinate the World | first=Yossi | last=Sheffi | publisher=MIT CTL Media | year=2021 | isbn=979-8-9850705-2-1 | url=https://books.google.com/books?id=ItdGEAAAQBAJ&pg=PA2006 }}</ref>
===Culture=== [[File:Easter Egg Hunt (5623253840).jpg|thumb|upright|Chocolate Easter eggs hidden as part of an egg hunt]] Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths.<ref>{{Cite web | last=Hall | first=Stephanie | date=6 April 2017 | 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 decoration is a common practice in many cultures worldwide. Christians view Easter eggs 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=2 April 2012 | work=Huffington Post | access-date=2018-03-31 | language=en-AU }}</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>{{cite book | chapter=Tradition without History | first=Jonas | last=Frykman | title=Treat or Trick? Halloween in a Globalising World | series=EBSCO ebook academic collection | editor1-first=Malcolm | editor1-last=Foley | editor2-first=Hugh | editor2-last=O’Donnell | publisher=Cambridge Scholars Publishing | year=2008 | isbn=978-1-4438-0265-9 | page=133 | chapter-url=https://books.google.com/books?id=oKsLBwAAQBAJ&pg=PA133 }}</ref> 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=5 October 2015 | 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 | last=Mcinnes | first=Anita | language=en-US | access-date=2018-03-31 | date=9 March 2017 | 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> 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>{{Cite web | url=https://australianmuseum.net.au/image/egg-specimens | title=Egg specimens | website=Australian Museum | date=29 April 2009 | first=Michael | last=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=25 March 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= March 31, 2018 }}</ref>
==See also== {{Div col}} *List of egg topics *Animal shell *Butterfly eggs *Egg white *Fossil egg *Haugh unit *Oology *Oviparous *Trophic egg * Organic egg production {{div col end}}
==References== {{reflist}}
==External links== {{Wiktionary}}
*{{wikiquote-inline}} *{{commons category-inline}}
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{{DEFAULTSORT:Egg (Biology)}} Category:Eggs Category:Animal reproductive system Category:Aviculture Category:Bird breeding Category:Oology