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Egg (biology)

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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.


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

non-avian 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 cells".

Eggs of different animal groups

Several major groups of animals typically have readily distinguishable eggs.

Overview of eggs from various animals
Class Types of eggs Development
Jawless fish Mesolecithal eggs, especially large in hagfish[3]
Larval stage in lampreys, direct development in hagfish.[4][5]
Cartilaginous fish Macrolecithal eggs with egg capsule[3] Direct development, viviparity in some species[6]
Bony fish Macrolecithal eggs, small to medium size, large eggs in the coelacanth[7] Larval stage, ovovivipary in some species.[8]
Amphibians Medium-sized mesolecithal eggs in all species.[7] Tadpole stage, direct development in some species.[7]
Large macrolecithal eggs, develop independent of water.[9] Direct development, some ovoviviparious
Large to very large macrolecithal eggs in all species, develop independent of water.[3] The young more or less fully developed, no distinct larval stage.
Macrolecithal eggs in
placental mammals.[3]
Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals.

Fish and amphibian eggs

yolk D. oil globule E. perivitelline space F. embryo

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

intrauterine cannibalism

In certain scenarios, some fish such as the

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

Chiromantis xerampelina

Bird eggs

Bird eggs are laid by females and

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


The default color of vertebrate eggs is the white of the

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 colors.

However some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid-state lubricant.[10] 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.[11]

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

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.


Bird eggshells are diverse. For example:

Tiny pores in bird eggshells allow the embryo to breathe. The domestic hen's egg has around 7000 pores.[12]

Some bird eggshells have a coating of

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.[13]


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.[14][15] 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.[16]


Many animals feed on eggs. For example, principal predators of the

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

Amniote eggs and embryos

Turtle eggs in a nest dug by a female common snapping turtle (Chelydra serpentina

Like amphibians,

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

("live birth").

Dinosaurs laid eggs, some of which have been preserved as petrified fossils.

Among mammals, early extinct species laid eggs, as do

mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.

Mammalian eggs

The eggs of the egg-laying mammals (the platypus and the echidnas) are macrolecithal eggs very much like those of reptiles. The eggs of marsupials are likewise macrolecithal, but rather small, and develop inside the body of the female, but do not form a placenta. The young are born at a very early stage, and can be classified as a "larva" in the biological sense.[17]


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

Invertebrate eggs

Eggs are common among


Evolution and structure

All sexually reproducing life, including both plants and animals, produces

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.[18]


The cycle of the egg's formation is started by the

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

animal kingdom


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.[19]

Microlecithal eggs require minimal yolk mass. Such eggs are found in

echinoderms, the lancelet and in most marine arthropods.[20] 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 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][20] 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]


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][21]

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.

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.[22]

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:[23]

The term hemotropic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.[28]

Human use


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

According to the

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.[29]

Vaccine manufacture

Many vaccines for infectious diseases are produced in fertile chicken eggs. The basis of this technology was the discovery in 1931 by

Rocky mountain spotted fever
and other diseases.


Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths.

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.[32]

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.[33]


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.[34]

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.[35] On the other hand, ongoing underground trading is becoming a serious issue.[36]

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,[37] and the collection in Western Australia Museum has been archived in a gallery.[38] 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.[39]


See also


  1. ^ "Whale Shark – Cartilaginous Fish". SeaWorld Parks & Entertainment. Archived from the original on 2014-06-09. Retrieved 2014-06-26.
  2. from the original on 10 May 2016.
  3. ^ a b c d e f g Hildebrand, M. & Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York City
  4. ^
    S2CID 198158310
  5. ^ Hardisty, M. W., and Potter, I. C. (1971). The Biology of Lampreys 1st ed. (Academic Press Inc.).
  6. . OCLC 156157504.
  7. ^ a b c d e f g Romer, A. S. & Parsons, T. S. (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia.
  8. ^ a b Stewart J. R. (1997): Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291–326. London: Academic Press.
  9. ^ Solomon, S.E. (1987). Egg shell pigmentation. In Egg Quality: Current Problems and Recent Advances (eds R.G. Wells & C.G. Belyarin). Butterworths, London, pp. 147–157.
  10. .
  11. ^ "The Parts of the Egg". Archived from the original on November 23, 2016.
  12. .
  13. .
  14. ^ Young, Ed (22 June 2017). "Why Are Bird Eggs Egg-Shaped? An Eggsplainer". The Atlantic. Archived from the original on 24 June 2017. Retrieved 23 June 2017.
  15. ^ Yutaka Nishiyama (2012). "The Mathematics of Egg Shape" (PDF). International Journal of Pure and Applied Mathematics. 78 (5): 679–689.
  16. .
  17. .
  18. ^ a b Barns, R.D. (1968): Invertebrate Zoology. W. B. Saunders Company, Philadelphia. 743 pages
  19. ^ Nixon, M. & Messenger, J.B (eds) (1977): The Biology of Cephalopods. Symposium of the Zoological Society of London, pp 38–615
  20. ^ Fricke, H.W. & Frahm, J. (1992): Evidence for lecithotrophic viviparity in the living coelacanth. Naturwissenschaften no 79: pp. 476–479
  21. ^ Thierry Lodé 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris
  22. from the original on 1 November 2017. Retrieved 25 November 2014.
  23. .
  24. .
  25. ISBN 978-0125449014. Archived from the original on 1 November 2017. Retrieved 25 November 2014. {{cite book}}: |first1= has generic name (help
  26. ^ "Online Etymology Dictionary". Archived from the original on 2014-05-14. Retrieved 2013-07-27.
  27. ^ Jewish Virtual Library Archived 2013-01-17 at the Wayback Machine Kashrut: Jewish Dietary Laws
  28. ^ Hall, Stephanie (2017-04-06). "The Ancient Art of Decorating Eggs | Folklife Today". Retrieved 2021-02-16.
  29. ^ Barooah, Jahnabi (2012-04-02). "Easter Eggs: History, Origin, Symbolism And Traditions (PHOTOS)". Huffington Post. Retrieved 2018-03-31.
  30. ^ L'ou com balla Archived 2016-04-09 at the Wayback Machine, Barcelona Cathedral.
  31. ^ Ramaswamy, Chitra (2015-10-05). "Beyond a yolk: a brief history of egging as a political protest". The Guardian. Retrieved 2018-03-31.
  32. ^ "Collecting bird eggs". 2017-03-09. Retrieved 2018-03-31.
  33. ^ "Protection of Birds Act 1954". Retrieved 2018-03-31.
  34. ^ Wheeler, Timothy (2015-03-16), Poached, retrieved 2018-03-31
  35. ^ "Egg specimens - Australian Museum". Retrieved 2018-03-31.
  36. ^ "Explore our Egg Collection | Western Australian Museum". Western Australian Museum. Retrieved 2018-03-31.
  37. ^ Golembiewski, Kate. "The Lost Victorian Art of Egg Collecting". The Atlantic. Retrieved 2018-03-31.

External links