An egg is the organic vessel containing the zygote in which an animal
embryo develops until it can survive on its own; at which point the
animal hatches. An egg results from fertilization of an ovum. Most
arthropods, vertebrates, and mollusks lay eggs, although some, such as
scorpions and most mammals, 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.
The largest recorded egg is from a whale shark, and was 30 cm
× 14 cm × 9 cm (11.8 in × 5.5 in
× 3.5 in) in size.
Whale shark eggs typically hatch
within the mother. At 1.5 kg (3.3 lb) and up to 17.8 cm
× 14 cm (7.0 in × 5.5 in), the ostrich egg
is the largest egg of any living bird, though the extinct elephant
bird and some dinosaurs laid larger eggs. The bee hummingbird produces
the smallest known bird egg, which weighs half of a gram (around
0.02 oz). Some eggs laid by reptiles and most fish, amphibians,
insects and other invertebrates can be even smaller.
Reproductive structures similar to the egg in other kingdoms are
termed "spores," or in spermatophytes "seeds," or in gametophytes "egg
1 Eggs of different animal groups
Fish and amphibian eggs
1.2.5 Various examples
Amniote eggs and embryos
1.4 Mammalian eggs
Evolution and structure
3 Scientific classifications
Egg size and yolk
3.2 Egg-laying reproduction
4 Human use
4.1.1 Eggs and Kashrut
4.2 Vaccine manufacture
7 See also
Eggs of different animal groups
Further information: egg cell
Several major groups of animals typically have readily distinguishable
Overview of eggs from various animals
Types of eggs
Mesolecithal eggs, especially large in hagfish
Larval stage in lampreys, direct development in hagfish.
Macrolecithal eggs with egg capsule
Direct development, viviparity in some species
Macrolecithal eggs, small to medium size, large eggs in the
Larval stage, ovovivipary in some species.
Medium-sized mesolecithal eggs in all species.
Tadpole stage, direct development in some species.
Large macrolecithal eggs, develop independent of water.
Direct development, some ovoviviparious
Large to very large macrolecithal eggs in all species, develop
independent of water.
The young more or less fully developed, no distinct larval stage.
Macrolecithal eggs in monotremes and marsupials, extreme microlecithal
eggs in placental mammals.
Young little developed with indistinct larval stage in monotremes and
marsupials, direct development in placentals.
Fish and amphibian eggs
Ichthyoplankton and Spawn (biology)
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.
Diagram of a fish egg: A. vitelline membrane B. chorion C. yolk D. oil
globule E. perivitelline space F. embryo
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 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 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
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.
The eggs of fish and amphibians are jellylike. Cartilagenous 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
Main article: bird egg
Bird eggs are laid by females and incubated for a time that varies
according to the species; a single young hatches from each egg.
Average clutch sizes range from one (as in condors) to about 17 (the
grey partridge). Some birds lay eggs even when not fertilized (e.g.
hens); it is not uncommon for pet owners to find their lone bird
nesting on a clutch of unfertilized eggs, which are sometimes called
The default color of vertebrate eggs is the white of the calcium
carbonate from which the shells are made, but some birds, mainly
passerines, produce colored eggs. The pigment biliverdin and its zinc
chelate give a green or blue ground color, and protoporphyrin produces
reds and browns as a ground color or as spotting.
Non-passerines typically have white eggs, except in some
ground-nesting groups such as the Charadriiformes, sandgrouse and
nightjars, where camouflage is necessary, and some parasitic cuckoos
which have to match the passerine host's egg. Most passerines, in
contrast, lay colored eggs, even if there is no need of cryptic
However some have suggested that the protoporphyrin markings on
passerine eggs actually act to reduce brittleness by acting as a solid
state lubricant. If there is insufficient calcium available in the
local soil, the egg shell may be thin, especially in a circle around
the broad end.
Protoporphyrin speckling compensates for this, and
increases inversely to the amount of calcium in the soil.
For the same reason, later eggs in a clutch are more spotted than
early ones as the female's store of calcium is depleted.
The color of individual eggs is also genetically influenced, and
appears to be inherited through the mother only, suggesting that the
gene responsible for pigmentation is on the sex determining W
chromosome (female birds are WZ, males ZZ).
It used to be thought that color was applied to the shell immediately
before laying, but this 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.
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 domestic
hen's egg has around 7000 pores.
Most bird eggs have an oval shape, with one end rounded and the other
more pointed. This shape results from the egg being forced through the
oviduct. Muscles contract the oviduct behind the egg, pushing it
forward. The egg's wall is still shapeable, and the pointed end
develops at the back. Long, pointy eggs are an incidental consequence
of having a streamlined body typical of birds with strong flying
abilities; flight narrows the oviduct, which changes the type of egg a
bird can lay. Cliff-nesting birds often have highly conical eggs.
They are less likely to roll off, tending instead to roll around in a
tight circle; this trait is likely to have arisen due to evolution via
natural selection. In contrast, many hole-nesting birds have nearly
Many animals feed on eggs. For example, principal predators of the
black oystercatcher's eggs include raccoons, skunks, mink, river and
sea otters, gulls, crows and foxes. The stoat (Mustela erminea) and
long-tailed weasel (M. frenata) steal ducks' eggs. Snakes of the
Elachistodon specialize in eating eggs.
Brood parasitism occurs in birds when one species lays its eggs in the
nest of another. In some cases, the host's eggs are removed or eaten
by the female, or expelled by her chick. Brood parasites include the
cowbirds and many Old World cuckoos.
An average whooping crane egg is 102 mm (4.0 in) long and
weighs 208 g (7.3 oz)
Eurasian oystercatcher eggs camouflaged in the nest
Egg of a senegal parrot, a bird that nests in tree holes, on a
1 cm (0.39 in) grid
Eggs of ostrich, emu, kiwi and chicken
Finch egg next to American dime
Eggs of duck, goose, guineafowl and chicken
Eggs of ostrich, cassowary, chicken, flamingo, pigeon and blackbird
Egg of an emu
Amniote eggs and embryos
Turtle eggs in a nest dug by a female common snapping turtle (Chelydra
Like amphibians, amniotes are air-breathing vertebrates, but they have
complex eggs or embryos, including an amniotic membrane. Amniotes
include reptiles (including dinosaurs and their descendants, birds)
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
Among mammals, early extinct species laid eggs, as do platypuses and
echidnas (spiny anteaters). Platypuses and two genera of echidna are
Marsupial and placental mammals do not lay
eggs, but their unborn young do have the complex tissues that identify
The eggs of the egg-laying mammals (the platypus and the echidnas) are
macrolecithal eggs very much like those of reptiles. The eggs of
marsupials are likewise macrolecithal, but rather small, and develop
inside the body of the female, but do not form a placenta. The young
are born at a very early stage, and can be classified as a "larva" in
the biological sense.
In placental mammals, the egg itself is void of yolk, but develops an
umbilical cord from structures that in reptiles would form the yolk
sac. Receiving nutrients from the mother, the fetus completes the
development while inside the uterus.
Orange-peel doris Acanthodoris lutea in tide pool laying
Eggs are common among invertebrates, including insects, spiders,
mollusks, and crustaceans.
Evolution and structure
All sexually reproducing life, including both plants and animals,
produces gametes. The male gamete cell, sperm, is usually motile
whereas the female gamete cell, the ovum, is generally larger and
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 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.
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
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. This
classification of eggs is based on the eggs of chordates, though the
basic principle extends to the whole animal kingdom.
Microlecithal eggs from the roundworm Toxocara
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.
Microlecithal eggs require minimal yolk mass. Such eggs are found in
flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet
and in most marine arthropods. In anatomically simple animals,
such as cnidarians and flatworms, the fetal development can be quite
short, and even microlecithal eggs can undergo direct development.
These small eggs can be produced in large numbers. In animals with
high egg mortality, microlecithal eggs are the norm, as in bivalves
and marine arthropods. However, the latter are more complex
anatomically than e.g. flatworms, and the small microlecithal eggs do
not allow full development. Instead, the eggs hatch into larvae, which
may be markedly different from the adult animal.
In placental mammals, where the embryo is nourished by the mother
throughout the whole fetal period, the egg is reduced in size to
essentially a naked egg cell.
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.
The larger yolk content of the mesolecithal eggs allows for a longer
fetal development. Comparatively anatomically simple animals will be
able to go through the full development and leave the egg in a form
reminiscent of the adult animal. This is the situation found in
hagfish and some snails. Animals with smaller size eggs or more
advanced anatomy will still have a distinct larval stage, though the
larva will be basically similar to the adult animal, as in lampreys,
coelacanth and the salamanders.
A baby tortoise begins to emerge "fully developed" from its
Eggs with a large yolk are called macrolecithal. The eggs are usually
few in number, and the embryos have enough food to go through full
fetal development in most groups. Macrolecithal eggs are only found
in selected representatives of two groups: Cephalopods and
Macrolecithal eggs go through a different type of development than
other eggs. Due to the large size of the yolk, the cell division can
not split up the yolk mass. The fetus instead develops as a plate-like
structure on top of the yolk mass, and only envelopes it at a later
stage. A portion of the yolk mass is still present as an external
or semi-external yolk sac at hatching in many groups. This form of
fetal development is common in bony fish, even though their eggs can
be quite small. Despite their macrolecithal structure, the small size
of the eggs does not allow for direct development, and the eggs hatch
to a larval stage ("fry"). In terrestrial animals with macrolecithal
eggs, the large volume to surface ratio necessitates structures to aid
in transport of oxygen and carbon dioxide, and for storage of waste
products so that the embryo does not suffocate or get poisoned from
its own waste while inside the egg, see amniote.
In addition to bony fish and cephalopods, macrolecithal eggs are found
in cartilaginous fish, reptiles, birds and monotreme mammals. The
eggs of the coelacanths can reach a size of 9 cm in diameter, and
the young go through full development while in the uterus, living on
the copious yolk.
Animals are commonly classified by their manner of reproduction, at
the most general level distinguishing egg-laying (Latin. oviparous)
from live-bearing (Latin. viviparous).
These classifications are divided into more detail according to the
development that occurs before the offspring are expelled from the
adult's body. Traditionally:
Ovuliparity means the female 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 diminiutive
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).
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
Marsupials excrete a "uterine milk" supplementing the
nourishment from the yolk sak.
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. In some hylid frogs, the embryo is fed by the
mother through specialized gills.
The term hemotropic derives from the Latin for blood-feeding,
contrasted with histotrophic for tissue-feeding.
Eggs laid by many different species, including birds, reptiles,
amphibians, and fish, have probably been eaten by mankind for
millennia. Popular choices for egg consumption are chicken, duck, roe,
and caviar, but by a wide margin the egg most often humanly consumed
is the chicken egg, typically unfertilized.
Eggs and Kashrut
Kashrut § Pareve foods, and
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). Kosher
meat and milk (or derivatives) cannot be mixed (Deuteronomy 14:21) or
stored together. Eggs are considered pareve (neither meat nor dairy)
despite being an animal product and can be mixed with either milk or
kosher meat. Mayonnaise, for instance, is usually marked "pareve"
despite by definition containing egg.
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
Ernest William Goodpasture at Vanderbilt University
that the rickettsia and viruses that cause a variety of diseases will
grow in chicken embryos. This enabled the development of vaccines
against influenza, chicken pox, smallpox, yellow fever, typhus, Rocky
mountain spotted fever and other diseases.
The egg is a symbol of new life and rebirth in many cultures around
the world. Christians view
Easter egg as a symbolic of the
resurrection of Jesus Christ.  A popular Easter tradition in some
parts of the world is the decoration of hard-boiled eggs (usually by
dyeing, but often by spray-painting). Adults often hide the eggs for
children to find, an activity known as an
Easter egg hunt. A similar
tradition of egg painting exists in areas of the world influenced by
the culture of Persia. Before the spring equinox in the Persian New
Year tradition (called Norouz), each family member decorates a
hard-boiled egg and sets them together in a bowl. The tradition of a
dancing egg is held during the feast of Corpus Christi in Barcelona
and other Catalan cities since the 16th century. It consists of an
emptied egg, positioned over the water jet from a fountain, which
starts turning without falling.
Although being a food item, 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 causing serious eye injury. On Halloween, for example, trick
or treaters have been known to throw eggs (and sometimes flour) at
property or people from whom they received nothing. Eggs are also
often thrown in protests, as they are inexpensive and nonlethal, yet
very messy when broken. 
Egg collecting was once a popular hobby. White settlers favored this
practice when they first came to Australia. Traditionally, the embryo
would be removed before a collector stored the egg shell. 
Collecting eggs of wild birds is now banned by many countries and
regions in consideration of the threaten to rare species. In the
United Kingdom, Protection of
Birds Act 1954 and Wildlife and
Countryside Act 1981 both state that if a person intentionally takes
or destroys an egg of any wild bird, he shall be considered guilty and
penalized.  On the other hand, ongoing underground trading is
becoming a serious issue. In the documentary film Poached (2015) ,
director Timothy Wheeler got the opportunity to work with the U.K.’s
Royal Society for the Protection of
Birds (RSPB) and the country’s
National Wildlife Crime Unit, and delved into the motivation behind
egg-collecting criminals. 
Since the protection of wild bird eggs was regulated, early
collections have come to the museums as curiosities. For example, the
Australian Museum hosts a collection of about 20,000 registered
clutches of eggs,  and the collection in Western Australia Museum
has been archived in a gallery. Scientists regard egg collections
as a good natural-history data, the details recorded in the
collectors' notes have helped them to understand birds' nesting
Insect eggs, in this case those of the Emperor gum moth, are often
laid on the underside of leaves.
Fish eggs, such as these herring eggs are often transparent and
fertilized after laying.
Skates and some sharks have a uniquely shaped egg case called a
Testudo hermanni emerging fully developed from a reptilian egg.
Schistosoma mekongi egg.
Eggs of Huffmanela hamo, a nematode parasite in a fish
Eggs of various parasites (mainly nematodes) from wild primates
List of egg topics
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Wikiquote has quotations related to: Eggs
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List of egg topics
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Origin of birds
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Evolution of birds
Families and orders
Glossary of bird terms
List by population
Lists by region
Recently extinct birds
Late Quaternary prehistoric birds
Casuariiformes (emus and cassowaries)
Phasianinae (pheasants and relatives)
Columbiformes (doves and pigeons)
Caprimulgiformes (nightjars and relatives)
Apodiformes (swifts and hummingbirds)
Charadriiformes (gulls and relatives)
Gruiformes (cranes and relatives)
Eurypygiformes (kagu and sunbittern)
Gaviiformes (loons or divers)
Procellariiformes (albatrosses and petrels)
Suliformes (cormorants and relatives)
Pelecaniformes (pelicans and relatives)
Cariamiformes (seriemas and relatives)
Falconiformes (falcons and relatives)
Passeriformes (perching birds)
Cathartiformes (New World vultures and condors)
Accipitriformes (eagles and hawks)
Trogoniformes (trogons and quetzals)
Leptosomatiformes (cuckoo roller)
Bucerotiformes (hornbills and hoopoes)
Coraciiformes (kingfishers and rollers)
Piciformes (woodpeckers and relatives)