Egg

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From Wikipedia, the free encyclopedia

This article is about biological eggs. For eggs as food,
see Eggs as food. For other uses, see Egg
(disambiguation).
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 Eggs of various birds, a reptile,
various cartilaginous fish, a cuttlefish and
various butterflies and moths. (Click on image for key)

Diagram of a fertilized chicken egg in

its ninth day. Membranes: allantois, chorion, amnion, and

vitellus/yolk. Six commercial chicken

eggs — view from the top against a white background
An egg is an organic vessel grown by an animal to carry
a possibly fertilized egg cell (a zygote) and
to incubate from it an embryo within the egg until the
embryo has become an animal fetus that can survive on
its own, at which point the animal hatches.
Most 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.
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.
[1]
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,[2]: 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 6.35–11.4 millimetres (0.250–
0.449 in) long and weighs half of a gram (around
0.02 oz).[2]: 132 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
cells".
Eggs of different animal groups
Further information: Egg cell
Several major groups of animals typically have readily
distinguishable eggs.
Overview of eggs from various animals

Class Types of eggs Development

Larval stage
Mesolecithal eggs,
Jawless in lampreys, direct
especially large
fish development in
in hagfish[3]
hagfish.[4][5][page needed]

Direct
Cartilagin Macrolecithal eggs development, viviparit
ous fish with egg capsule[3] y in some species[6]
[page needed]

Macrolecithal eggs, small

Larval stage, ovovivip
Bony fish to medium size, large
ary in some species.[8]
eggs in the coelacanth [7]

Medium-sized Tadpole stage, direct

Amphibia
mesolecithal eggs in all development in some
ns
species.[7] species.[7]
 Large macrolecithal eggs,
Direct development,
Reptiles develop independent of
some ovoviviparious
water.[9]

Large to very large

The young more or
macrolecithal eggs in all
Birds less fully developed,
species, develop
no distinct larval stage.
independent of water.[3]

Young little developed

Macrolecithal eggs
with indistinct larval
in monotremes and marsu
stage in monotremes
Mammals pials, extreme
and marsupials, direct
microlecithal eggs
development in
in placental mammals.[3]
placentals.
Fish and amphibian eggs
"Fish egg" redirects here. For fish eggs as food,
see Roe.
See also: 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 intrauterine cannibalism.
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.

Moor frog eggs utilize glycans to

form a hydrophilic jelly coat that protects the egg
The eggs of fish and amphibians are jellylike.
Cartilaginous fish (sharks, skates, rays, chimaeras) eggs
are fertilized internally and exhibit a wide variety of both
internal and external embryonic development. Most fish
species spawn eggs that are fertilized externally,
typically with the male inseminating the eggs after the
female lays them. These eggs do not have a shell and
would dry out in the air. Even air-breathing amphibians
lay their eggs in water, or in protective foam as with the
Coast foam-nest treefrog, Chiromantis xerampelina.
Bird eggs
Main 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 wind-eggs.
Colours

Guillemot eggs
The default colour of vertebrate eggs is the white of
the calcium carbonate from which the shells are made,
but some birds, mainly passerines, produce coloured
eggs. The colour comes from pigments deposited on top
of the calcium carbonate base; biliverdin and its
zinc chelate, and bilirubin, give a green or blue ground
colour, while protoporphyrin IX produces reds and
browns as a ground colour or as spotting.[10]
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
coloured eggs, even if there is no need of cryptic colors.
However, some have suggested that the protoporphyrin
markings on passerine eggs actually act to reduce
brittleness by acting as a solid-state lubricant.[11] 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.[12]
For the same reason, later eggs in a clutch are more
spotted than early ones as the female's store of calcium
is depleted.
The color of individual eggs is also genetically
influenced, and appears to be inherited through the
mother only, suggesting that the gene responsible for
pigmentation is on the sex-determining W
chromosome (female birds are WZ, males ZZ).
It used to be thought that color was applied to the shell
immediately before laying, but subsequent research
shows that coloration is an integral part of the
development of the shell, with the same protein
responsible for depositing calcium carbonate, or
protoporphyrins when there is a lack of that mineral.
In species such as the common guillemot, which nest in
large groups, each female's eggs have very different
markings, making it easier for females to identify their
own eggs on the crowded cliff ledges on which they
breed.
Yolks of birds' eggs are yellow from carotenoids, it is
affected by their living conditions and diet.[10]
Shell

Bird eggshells are diverse. For example:

 cormorant eggs are rough and chalky
 tinamou eggs are shiny

 duck eggs are oily and waterproof

 cassowary eggs are heavily pitted

Tiny pores in bird eggshells allow the embryo to breathe.

The domestic hen's egg has around 7000 pores.[13]
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.
[14]

Shape

A 3D model of an egg
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.[citation needed]. 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.[15][16]
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.[17]
Predation

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
genera Dasypeltis and Elachistodon specialize in eating
eggs.
Brood parasitism occurs in birds when one species lays
its eggs in the nest of another. In some cases, the host's
eggs are removed or eaten by the female, or expelled by
her chick. Brood parasites include the cowbirds and
many Old World cuckoos.
Various examples

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 blackbir
d

Egg of an emu
 

Egg from a chicken compared to a 1 euro coin, great tit egg

and a corn grain

Bird nest with brown marbling eggs of a robin

A spherical chicken egg

Amniote eggs and embryos

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. Amniotes
include reptiles (including dinosaurs and their
descendants, birds) and mammals.
Reptile eggs are often rubbery and are always initially
white. They are able to survive in the air. Often the sex of
the developing embryo is determined by the temperature
of the surroundings, with cooler temperatures favouring
males. Not all reptiles lay eggs; some
are viviparous ("live birth").
Dinosaurs laid eggs, some of which have been
preserved as petrified fossils.
Among mammals, early extinct species laid eggs, as
do platypuses and echidnas (spiny anteaters).
Platypuses and two genera of echidna are
Australian monotremes. Marsupial and placental mamm
als do not lay eggs, but their unborn young do have the
complex tissues that identify amniotes.
Mammalian eggs
The eggs of the egg-laying mammals (the platypus and
the echidnas) are macrolecithal eggs very much like
those of reptiles. The eggs of marsupials are likewise
macrolecithal, but rather small, and develop inside the
body of the female, but do not form a placenta. The
young are born at a very early stage, and can be
classified as a "larva" in the biological sense.[18]
In placental mammals, the egg itself is void of yolk, but
develops an umbilical cord from structures that in reptiles
would form the yolk sac. Receiving nutrients from the
mother, the fetus completes the development while
inside the uterus.
Invertebrate eggs

Orange-peel doris (Acanthodoris

lutea), a nudibranch, in tide pool laying eggs
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.[19]
Formation
The cycle of the egg's formation is started by
the gamete ovum being released (ovulated) and egg
formation being started. 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.
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.[7] This
classification of eggs is based on the eggs of chordates,
though the basic principle extends to the whole animal
kingdom.
Microlecithal
 Microlecithal eggs from

the roundworm Toxocara Microlecith

al 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.[20]
Microlecithal eggs require minimal yolk mass. Such eggs
are found
in flatworms, roundworms, annelids, bivalves, echinoder
ms, the lancelet and in most marine arthropods.[21] In
anatomically simple animals, such as cnidarians and
flatworms, the fetal development can be quite short, and
even microlecithal eggs can undergo direct development.
These small eggs can be produced in large numbers. In
animals with high egg mortality, microlecithal eggs are
the norm, as in bivalves and marine arthropods.
However, the latter are more complex anatomically than
e.g. flatworms, and the small microlecithal eggs do not
allow full development. Instead, the eggs hatch
into larvae, which may be markedly different from the
adult animal.
In placental mammals, where the embryo is nourished by
the mother throughout the whole fetal period, the egg is
reduced in size to essentially a naked egg cell.
Mesolecithal

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.[3]
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.[4][21] 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.[3]
Macrolecithal

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.[7] Macrolecithal eggs are only found in selected
representatives of two
groups: Cephalopods and vertebrates.[7][22]
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.[7] 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.[9]
In addition to bony fish and cephalopods, macrolecithal
eggs are found in cartilaginous
fish, reptiles, birds and monotreme mammals.[3] The eggs
of the coelacanths can reach a size of 9 cm (3.5 in) in
diameter, and the young go through full development
while in the uterus, living on the copious yolk.[23]
Egg-laying reproduction
Animals are commonly classified by their manner of
reproduction, at the most general level distinguishing
egg-laying (Latin. oviparous) from live-bearing
(Latin. viviparous).
These classifications are divided into more detail
according to the development that occurs before the
offspring are expelled from the adult's body.
Traditionally:[24]
 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.[25]
 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.[26][27] In some hylid
frogs, the embryo is fed by the mother through
specialized gills.[28]
The term hemotrophic derives from the Latin for blood-
feeding, contrasted with histotrophic for tissue-feeding.[29]
Human use
Food
Main article: Eggs as food

Wikibooks Cookbook has a recipe/module on

 Egg
Eggs laid by many different species, including birds,
reptiles, amphibians, and fish, have probably been eaten
by people for millennia. Popular choices for egg
consumption are chicken, duck, roe, and caviar, but by a
wide margin the egg most often humanly consumed is
the chicken egg, typically unfertilized.
Eggs and Kashrut
See also: Kashrut § Pareve foods, 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.[30]
Vaccine manufacture
Many vaccines for infectious diseases are produced in
fertile chicken eggs. The basis of this technology was the
discovery in 1931 by Alice Miles Woodruff and Ernest
William Goodpasture at Vanderbilt University that
the rickettsia and viruses that cause a variety of
diseases will grow in chicken embryos. This enabled the
development of vaccines against influenza, chicken
pox, smallpox, yellow fever, typhus, Rocky mountain
spotted fever and other diseases.
Culture

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.
[31]
Egg decoration is a common practice in many cultures
worldwide. Christians view Easter eggs as symbolic of
the resurrection of Jesus Christ.[32] A popular Easter
tradition in some parts of the world is the decoration of
hard-boiled eggs (usually by dyeing, but often by hand-
painting or spray-painting). Adults often hide the eggs for
children to find, an activity known as an Easter egg hunt.
A similar tradition of egg painting exists in areas of the
world influenced by the culture of Persia. Before the
spring equinox in the Persian New Year tradition (called
Norouz), each family member decorates a hard-boiled
egg and sets them together in a bowl. The tradition of
a dancing egg is held during the feast of Corpus Christi
in Barcelona and other Catalan cities since the 16th
century. It consists of an emptied egg, positioned over
the water jet from a fountain, which starts turning without
falling.[33]
Although a food item, raw eggs are sometimes thrown at
houses, cars, or people. This act, known commonly as
"egging" in the various English-speaking countries, is a
minor form of vandalism and, therefore, usually a
criminal offense and is capable of damaging property
(egg whites can degrade certain types of vehicle paint)
as well as potentially causing serious eye injury. On
Halloween, for example, trick or treaters have been
known to throw eggs (and sometimes flour) at property
or people from whom they received nothing.[citation
needed]
Eggs are also often thrown in protests, as they are
inexpensive and nonlethal, yet very messy when broken.
[34]

Collecting
Main article: 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.[35]
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.[36] On the other hand, ongoing
underground trading is becoming a serious issue.[37]
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,
[38]
and the collection in Western Australia Museum has
been archived in a gallery.[39] 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.[40]
Gallery
 

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 mermaid's purse.
 

A Testudo hermanni emerging fully developed from a

reptilian egg.

A Schistosoma mekongi egg.

Eggs of Huffmanela hamo, a nematode parasite in a fish


Eggs of various parasites (mainly nematodes) from
wild primates