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An egg is an organic vessel grown by an animal to carry a possibly fertilization egg cell (a zygote) and to egg incubation 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 , vertebrates (excluding live-bearing mammals), and Mollusca lay eggs, although some, such as , do not.
Reptile eggs, , and monotreme eggs are laid out of water and are surrounded by a protective eggshell, 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 in size. Whale shark eggs typically hatch within the mother. At and up to , the ostrich egg is the largest egg of any living bird, 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 long and weighs half of a gram (around 0.02 oz). Some eggs laid by reptiles and most fish, amphibians, insects, and other can be even smaller.
Reproductive structures similar to the egg in other kingdoms are termed "", or in "", or in "egg cells".
| + Overview of eggs from various animals ! Class ! Types of eggs ! Development | ||
| Agnatha | Mesolecithal eggs, especially large in hagfish | Larval stage in , direct development in hagfish. |
| Chondrichthyes | Macrolecithal eggs with egg capsule | Direct development, viviparity in some species (1984). 9789251045435, Food and Agriculture Organization of the United Nations. ISBN 9789251045435 |
| Osteichthyes | Macrolecithal eggs, small to medium size, large eggs in the coelacanth | stage, Ovoviviparity in some species. |
| Medium-sized mesolecithal eggs in all species. | Tadpole stage, direct development in some species. | |
| Reptiles | Large macrolecithal eggs, develop independent of water. | Direct development, some ovoviviparious |
| Birds | Large to very large macrolecithal eggs in all species, develop independent of water. | The young more or less fully developed, no distinct larval stage. |
| Mammals | Macrolecithal eggs in and , extreme microlecithal eggs in placental mammals. | Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals. |
A few fish, notably the rays and most 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 Viviparity, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment.
The eggs of fish and 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.
Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and , where camouflage is necessary, and some brood parasite 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. 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 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.
of birds' eggs are yellow from , it is affected by their living conditions and diet.
Tiny pores in bird eggshells allow the embryo to breathe. The chicken's egg has around 7000 pores.
Some bird eggshells have a coating of vaterite , 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.
Cliff-nesting birds often have highly cone eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly sphere 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 and many Old World .
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 and (spiny anteaters). Platypuses and two genera of echidna are Australian monotremes. Marsupial and placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.
In , 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.
A recent proposal suggests that the phylotype animal originated in cell aggregates before the existence of an egg stage of development. Eggs, in this view, were later evolutionary emergence, selected for their role in ensuring genetic uniformity among the cells of incipient multicellular organisms.
Microlecithal eggs require minimal yolk mass. Such eggs are found in , , , , Echinodermata, the lancelet and in most marine .Barns, R.D. (1968): Invertebrate Zoology. W. B. Saunders Company, Philadelphia. 743 pages 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 , 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.
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 . 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 , coelacanth and the .
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.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.
In addition to bony fish and cephalopods, macrolecithal eggs are found in Chondrichthyes, , and monotreme mammals. The eggs of the can reach a size of in diameter, and the young go through full development while in the uterus, living on the copious yolk.Fricke, H.W. & Frahm, J. (1992): Evidence for lecithotrophic viviparity in the living coelacanth. Naturwissenschaften no 79: pp. 476–479
These classifications are divided into more detail according to the development that occurs before the offspring are expelled from the adult's body. Traditionally:Thierry Lodé 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris
The term hemotrophic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.
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. Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken.
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. 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, and the collection in Western Australia Museum has been archived in a gallery. 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.
Macrolecithal
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: and .Nixon, M. & Messenger, J.B (eds) (1977): The Biology of Cephalopods. Symposium of the Zoological Society of London, pp 38–615
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).
(1986). 9780125449014, Academic Press. ISBN 9780125449014
Human use
Food
Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia. Popular choices for egg consumption are chicken, duck, roe, and caviar, but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.
Eggs and Kashrut
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." Jewish Dietary Laws (Kashrut): Overview of Laws & Regulations", Jewish Virtual Library. .
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 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
Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths. Egg decorating is a common practice in many cultures worldwide. Christians view as 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 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 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.
Collecting
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.
See also
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