Egg

The egg is the organic vessel containing the zygote in which an embryo develops until it can survive on its own; at which point the animal hatches. An egg results from fertilization of an egg cell. Most arthropods, vertebrates (excluding 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] 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 cells".

Adolphe Millot oeufs-fixed
Eggs of various birds, a reptile, various cartilaginous fish, a cuttlefish and various butterflies and moths. (Click on image for key)
Chicken egg diagram
Diagram of a chicken egg in its 9th day. Membranes: allantois, chorion, amnion, and vitellus/ yolk.

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]
Reptiles Large macrolecithal eggs, develop independent of water.[9] Direct development, some ovoviviparious
Birds 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.
Mammals Macrolecithal eggs in monotremes and marsupials, extreme microlecithal eggs in placental mammals.[3] Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals.

Fish and amphibian eggs

Salmoneggskils
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.
Fish Egg Diagram (1)
Diagram of a fish egg: A. vitelline membrane B. chorion C. yolk D. oil globule E. perivitelline space F. embryo
Salmonlarvakils
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.

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 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.

Colors

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 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 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.

Shell

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 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.[13]

Shape

Oeuf.stl
A 3-D 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. 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] 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.[15]

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

Egg125o

An average whooping crane egg is 102 mm (4.0 in) long and weighs 208 g (7.3 oz)

Oystercatcher Eggs Norway

Eurasian oystercatcher eggs camouflaged in the nest

Senegal egg 10s06

Egg of a senegal parrot, a bird that nests in tree holes, on a 1 cm (0.39 in) grid

Comparison of eggs by Zureks

Eggs of ostrich, emu, kiwi and chicken

Finch Egg

Finch egg next to American dime

Ouă

Eggs of duck, goose, guineafowl and chicken

Emu Egg

Egg of an emu

Amniote eggs and embryos

Snapping turtle eggs md
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 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.[16]

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

Editing Image-Acanthodoris lutea laying eggs 2
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.[17]

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

Toxocara embryonated eggs
Microlecithal eggs from the roundworm Toxocara
Paragonimus westermani 01
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.[18]

Microlecithal eggs require minimal yolk mass. Such eggs are found in flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods.[19] 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 closeup
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][19] 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

Tortoise-Hatchling
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][20]

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 in diameter, and the young go through full development while in the uterus, living on the copious yolk.[21]

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:[22]

  • 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). 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 sak.[23]
  • 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.[24][25] In some hylid frogs, the embryo is fed by the mother through specialized gills.[26]

The term hemotropic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding.[27]

Human use

Food

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

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.[28]

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

The egg is a symbol of new life and rebirth in many cultures around the world. Christians view Easter eggs as symbolic of the resurrection of Jesus Christ.[29] 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.[30]

Although 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.[31]

Collecting

Egg collecting was a popular hobby in various contexts, including among the first Australians favored this practice. Traditionally, the embryo would be removed before a collector stored the egg shell.[32]

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, the practice is prohibited by the Protection of Birds Act 1954 and Wildlife and Countryside Act 1981.[33] On the other hand, ongoing underground trading is becoming a serious issue.[34]

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,[35] and the collection in Western Australia Museum has been archived in a gallery.[36] 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.[37]

Gallery

Emperor Gum Moth eggs2

Insect eggs, in this case those of the Emperor gum moth, are often laid on the underside of leaves.

Clupeaharenguskils2

Fish eggs, such as these herring eggs are often transparent and fertilized after laying.

Skate egg case (Raja binoculata) 01

Skates and some sharks have a uniquely shaped egg case called a mermaid's purse.

Éclosion tortue d'Hermann

A Testudo hermanni emerging fully developed from a reptilian egg.

Huffmanela hamo eggs (Microscope) 1D

Eggs of Huffmanela hamo, a nematode parasite in a fish

Parasite140080-fig3 Gastrointestinal parasites in seven primates of the Taï National Park - Helminths

Eggs of various parasites (mainly nematodes) from wild primates

See also

References

  1. ^ "Whale Shark – Cartilaginous Fish". SeaWorld Parks & Entertainment. Archived from the original on 2014-06-09. Retrieved 2014-06-26.
  2. ^ D.R. Khanna (1 January 2005). Biology of Birds. Discovery Publishing House. p. 130. ISBN 978-81-7141-933-3. Archived 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. ^ a b Gorbman, A. (June 1997). "Hagfish development". Zoological Journal. 14 (3): 375–390. doi:10.2108/zsj.14.375.
  5. ^ Hardisty, M. W., and Potter, I. C. (1971). The Biology of Lampreys 1st ed. (Academic Press Inc.).
  6. ^ Leonard J. V. Compagno (1984). Sharks of the World: An annotated and illustrated catalogue of shark species known to date. Food and Agriculture Organization of the United Nations. ISBN 92-5-104543-7. 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. ^ Peter Scott: Livebearing Fishes, p. 13. Tetra Press 1997. ISBN 1-56465-193-2
  9. ^ 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.
  10. ^ 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.
  11. ^ Gosler, Andrew G.; James P. Higham; S. James Reynolds (2005). "Why are birds' eggs speckled?". Ecology Letters. 8 (10): 1105–1113. doi:10.1111/j.1461-0248.2005.00816.x.
  12. ^ Vermont educational site Archived 2016-11-23 at the Wayback Machine
  13. ^ Portugal, J. P.; Bowen, J.; Riehl, C. (2018). "A rare mineral, vaterite, acts as a shock absorber in the eggshell of a communally nesting bird". Ibis. 160 (1): 173–178. doi:10.1111/ibi.12527.
  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. ^ Colbert, H.E & Morales, M. (1991): Evolution of the Vertebrates – A History of Backboned Animals Through Time. 4. utgave. John Wiely & sons inc, New York City. 470 pages ISBN 0-471-85074-8
  17. ^ Newman, S.A. (2011). "Animal egg as evolutionary innovation: a solution to the 'embryonic hourglass' puzzle". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 316 (7): 467–483. doi:10.1002/jez.b.21417. PMID 21557469.
  18. ^ Reitzel, A.M.; Sullivan, J.C; Finnery, J.R (2006). "Qualitative shift to indirect development in the parasitic sea anemone Edwardsiella lineata". Integrative and Comparative Biology. 46 (6): 827–837. doi:10.1093/icb/icl032. PMID 21672788. Archived from the original on 2016-08-08.
  19. ^ a b Barns, R.D. (1968): Invertebrate Zoology. W. B. Saunders Company, Philadelphia. 743 pages
  20. ^ Nixon, M. & Messenger, J.B (eds) (1977): The Biology of Cephalopods. Symposium of the Zoological Society of London, pp 38–615
  21. ^ Fricke, H.W. & Frahm, J. (1992): Evidence for lecithotrophic viviparity in the living coelacanth. Naturwissenschaften no 79: pp 476–479
  22. ^ Thierry Lodé 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris
  23. ^ USA, David O. Norris, Ph.D., Professor Emeritus, Department of Integrative Physiology, University of Colorado at Boulder, Colorado, USA, James A. Carr, Ph.D., faculty director, Joint Admission Medical Program, Department of Biological Sciences, Texas Tech University, Lubbock, Texas (2013). Vertebrate endocrinology (Fifth ed.). p. 349. ISBN 978-0123948151. Archived from the original on 1 November 2017. Retrieved 25 November 2014.
  24. ^ Hamlett, William C. (1989). "Evolution and morphogenesis of the placenta in sharks". Journal of Experimental Zoology. 252 (S2): 35–52. doi:10.1002/jez.1402520406.
  25. ^ Jerez, Adriana; Ramírez-Pinilla, Martha Patricia (November 2003). "Morphogenesis of extraembryonic membranes and placentation inMabuya mabouya (Squamata, Scincidae)". Journal of Morphology. 258 (2): 158–178. doi:10.1002/jmor.10138. PMID 14518010.
  26. ^ Gorbman, edited by Peter K.T. Pang, Martin P. Schreibman; consulting editor, Aubrey (1986). Vertebrate endocrinology : fundamentals and biomedical implications. Orlando: Academic Press. p. 237. ISBN 978-0125449014. Archived from the original on 1 November 2017. Retrieved 25 November 2014.CS1 maint: Extra text: authors list (link)
  27. ^ "Online Etymology Dictionary". Etymonline.com. Archived from the original on 2014-05-14. Retrieved 2013-07-27.
  28. ^ Jewish Virtual Library Archived 2013-01-17 at the Wayback Machine Kashrut: Jewish Dietary Laws
  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". echonewspaper.com.au. 2017-03-09. Retrieved 2018-03-31.
  33. ^ "Protection of Birds Act 1954". www.legislation.gov.uk. Retrieved 2018-03-31.
  34. ^ Wheeler, Timothy (2015-03-16), Poached, retrieved 2018-03-31
  35. ^ "Egg specimens - Australian Museum". australianmuseum.net.au. 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.
Amniote

Amniotes (from Greek ἀμνίον amnion, "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός amnos, "lamb") are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes lay their eggs on land or retain the fertilized egg within the mother, and are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the evolutionary great chain of being.

Amniotes are tetrapods (descendants of four-limbed and backboned animals) that are characterised by having an egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anamniotes (including frogs) typically do. Amniotes include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their ancestors, back to amphibians. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians.The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago, in the Carboniferous geologic period. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms.

The amniotic egg represents a critical divergence within the vertebrates, one enabling amniotes to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago).

Balut (food)

Balut ( bə-LOOT, BAH-loot; also spelled as balot) is a developing bird embryo (usually a duck) that is boiled and eaten from the shell. It originated from and is commonly sold as street food in the Philippines.

The Tagalog and Malay word balot means "wrapped". The length of incubation before the egg is cooked is a matter of local preference, but generally ranges between 14 and 21 days.

The eating of balut is controversial due to religious, animal welfare, and human health concerns.

Biryani

Biryani (pronounced [bɪr.jaːniː]), also known as biriyani, biriani, birani or briyani, is a mixed rice dish with its origins among the Muslims of the Indian subcontinent. This dish is especially popular throughout the Indian subcontinent, as well as among the diaspora from the region. It is also prepared in other regions such as Iraqi Kurdistan. It is made with Indian spices, rice, meat (chicken, goat, beef, prawn, or fish), vegetables or eggs.

Chicken or the egg

The chicken or the egg causality dilemma is commonly stated as "which came first: the chicken or the egg?". The dilemma stems from the observation that all chickens hatch from eggs and all chicken eggs are laid by chickens. "Chicken-and-egg" is a metaphoric adjective describing situations where it is not clear which of two events should be considered the cause and which should be considered the effect, to express a scenario of infinite regress, or to express the difficulty of sequencing actions where each seems to depend on others being done first. Plutarch posed the question as a philosophical matter in his essay "The Symposiacs", written in the 1st century CE.

Custard

Custard is a variety of culinary preparations based on milk or cream cooked with egg yolk to thicken it, and sometimes also flour, corn starch, or gelatin. Depending on the recipe, custard may vary in consistency from a thin pouring sauce (crème anglaise) to the thick pastry cream (crème pâtissière) used to fill éclairs. The most common custards are used in desserts or dessert sauces and typically include sugar and vanilla, however savory custards are also found, e.g. in quiche.

Custard is usually cooked in a double boiler (bain-marie), or heated very gently in a saucepan on a stove, though custard can also be steamed, baked in the oven with or without a water bath, or even cooked in a pressure cooker. Custard preparation is a delicate operation, because a temperature increase of 3–6 °C (5–10 °F) leads to overcooking and curdling. Generally, a fully cooked custard should not exceed 80 °C (~175 °F); it begins setting at 70 °C (~160 °F). A water bath slows heat transfer and makes it easier to remove the custard from the oven before it curdles. Adding a small amount of cornflour to the egg-sugar mixture stabilises the resulting custard, allowing it to be cooked in a single pan as well as in a double-boiler. A sous-vide water bath may be used to precisely control temperature.

Easter egg

Easter eggs, also called Paschal eggs, are eggs that are sometimes decorated. They are usually used as gifts on the occasion of Easter. As such, Easter eggs are common during the season of Eastertide (Easter season). The oldest tradition is to use dyed and painted chicken eggs, but a modern custom is to substitute chocolate eggs wrapped in colored foil, hand-carved wooden eggs, or plastic eggs filled with confectionery such as chocolate. However, real eggs continue to be used in Central and Eastern European tradition. Although eggs, in general, were a traditional symbol of fertility and rebirth, in Christianity, for the celebration of Eastertide, Easter eggs symbolize the empty tomb of Jesus, from which Jesus resurrected. In addition, one ancient tradition was the staining of Easter eggs with the colour red "in memory of the blood of Christ, shed as at that time of his crucifixion." This custom of the Easter egg can be traced to early Christians of Mesopotamia, and from there it spread into Russia and Siberia through the Orthodox Churches, and later into Europe through the Catholic and Protestant Churches. This Christian use of eggs may have been influenced by practices in "pre-dynastic period in Egypt, as well as amid the early cultures of Mesopotamia and Crete."

Easter egg (media)

In computer software and media, an Easter egg is an intentional inside joke, hidden message or image, or secret feature of a work. It is usually found in a computer program, video game, or DVD/Blu-ray Disc menu screen. The name is used to evoke the idea of a traditional Easter egg hunt. The term was coined to describe a hidden message in the Atari video game Adventure that encouraged the player to find further hidden messages in later games, leading them on a 'hunt'.

Egg as food

Some eggs are laid by female animals of many different species, including birds, reptiles, amphibians, mammals, and fish, and have been eaten by humans for thousands of years. Bird and reptile eggs consist of a protective eggshell, albumen (egg white), and vitellus (egg yolk), contained within various thin membranes. The most commonly consumed eggs are chicken eggs. Other poultry eggs including those of duck and quail also are eaten. Fish eggs are called roe and caviar.

Egg yolks and whole eggs store significant amounts of protein and choline, and are widely used in cookery. Due to their protein content, the United States Department of Agriculture formerly categorized eggs as Meats within the Food Guide Pyramid (now MyPlate). Despite the nutritional value of eggs, there are some potential health issues arising from cholesterol content, salmonella contamination, and allergy to egg proteins.

Chickens and other egg-laying creatures are kept widely throughout the world and mass production of chicken eggs is a global industry. In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens. There are issues of regional variation in demand and expectation, as well as current debates concerning methods of mass production. In 2012, the European Union banned battery husbandry of chickens.

Egg cell

The egg cell, or ovum (plural ova), is the female reproductive cell (gamete) in oogamous organisms. The egg cell is typically not capable of active movement, and it is much larger (visible to the naked eye) than the motile sperm cells. When egg and sperm fuse, a diploid cell (the zygote) is formed, which rapidly grows into a new organism.

Egg white

Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms around fertilized or unfertilized egg yolks. The primary natural purpose of egg white is to protect the yolk and provide additional nutrition for the growth of the embryo (when fertilized).

Egg white consists primarily of about 90% water into which about 10% proteins (including albumins, mucoproteins, and globulins) are dissolved. Unlike the yolk, which is high in lipids (fats), egg white contains almost no fat, and carbohydrate content is less than 1%. Egg whites contain about 56% of the protein in the egg. Egg white has many uses in food (e.g. meringue, mousse) and also many other uses (e.g. in the preparation of vaccines such as those for influenza).

Eggnog

Eggnog , egg nog or egg-nog, historically also known (when alcoholic beverages are added) as milk punch or egg milk punch, is a rich, chilled, sweetened, dairy-based beverage. It is traditionally made with milk, cream, sugar, whipped egg whites, and egg yolks (which gives it a frothy texture, and its name). In some contexts, distilled spirits such as brandy, rum, whisky or bourbon are added to the drink.

Throughout Canada and the United States, eggnog is traditionally consumed over the Christmas season, from late November until the end of the holiday season. Eggnog has also gained popularity in Australia. A variety called Ponche Crema has been made and consumed in Venezuela and Trinidad since the 1900s, also as part of the Christmas season. During that time, commercially prepared eggnog is sold in grocery stores in these countries.

Eggnog is also homemade using milk, eggs, sugar, and flavorings, and served with cinnamon or nutmeg. While eggnog is often served chilled, in some cases it is warmed, particularly on cold days (similar to the way mulled wine is served warm). Eggnog or eggnog flavoring may also be used in other drinks, such as coffee (e.g. an "eggnog latte" espresso drink) and tea, or to dessert foods such as egg-custard puddings.

Eggplant

Eggplant (US), aubergine (UK), or brinjal (South Asia and South Africa) is a plant species in the nightshade family Solanaceae, Solanum melongena, grown for its often purple edible fruit.

The spongy, absorbent fruit of the plant is widely used in cooking in many different cuisines, and is often considered a vegetable, even though it is a berry by botanical definition. As a member of the genus Solanum, it is related to the tomato and the potato. Like the tomato, its skin and seeds can be eaten, but, like the potato, it is not advisable to eat it raw. Eggplant is nutritionally low in macronutrient and micronutrient content, but the capability of the fruit to absorb oils and flavors into its flesh through cooking expands its use in the culinary arts.

It was originally domesticated from the wild nightshade species thorn or bitter apple, S. incanum, probably with two independent domestications: one in South Asia, and one in East Asia.

Fabergé egg

A Fabergé egg (Russian: Яйца Фаберже́, yaytsa faberzhe) is a jewelled egg (possibly numbering as many as 69, of which 57 survive today) created by the House of Fabergé, in St. Petersburg, Imperial Russia. Virtually all were manufactured under the supervision of Peter Carl Fabergé between 1885 and 1917, the most famous being the 50 "Imperial" eggs, 44 of which survive, made for the Russian Tsars Alexander III and Nicholas II as Easter gifts for their wives and mothers.

In vitro fertilisation

In vitro fertilisation (IVF) is a process of fertilisation where an egg is combined with sperm outside the body, in vitro ("in glass"). The process involves monitoring and stimulating a woman's ovulatory process, removing an ovum or ova (egg or eggs) from the woman's ovaries and letting sperm fertilise them in a liquid in a laboratory. After the fertilised egg (zygote) undergoes embryo culture for 2–6 days, it is implanted in the same or another woman's uterus, with the intention of establishing a successful pregnancy.

IVF is a type of assisted reproductive technology used for infertility treatment and gestational surrogacy. A fertilised egg may be implanted into a surrogate's uterus, and the resulting child is genetically unrelated to the surrogate. Some countries banned or otherwise regulate the availability of IVF treatment, giving rise to fertility tourism. Restrictions on the availability of IVF include costs and age, in order for a woman to carry a healthy pregnancy to term. IVF is generally not used until less invasive or expensive options have failed or been determined unlikely to work.

In 1978 Louise Brown was the first child successfully born after her mother received IVF treatment. Brown was born as a result of natural-cycle IVF, where no stimulation was made. The procedure took place at Dr Kershaw's Cottage Hospital (now Dr Kershaw's Hospice) in Royton, Oldham, England. Robert G. Edwards was awarded the Nobel Prize in Physiology or Medicine in 2010. The physiologist co-developed the treatment together with Patrick Steptoe and embryologist Jean Purdy but the latter two were not eligible for consideration as they had died and the Nobel Prize is not awarded posthumously.

With egg donation and IVF, women who are past their reproductive years, have infertile male partners, have idiopathic female-fertility issues, or have reached menopause, can still become pregnant. Adriana Iliescu held the record as the oldest woman to give birth using IVF and donated egg, when she gave birth in 2004 at the age of 66, a record passed in 2006. After the IVF treatment, some couples get pregnant without any fertility treatments. In 2018 it was estimated that eight million children had been born worldwide using IVF and other assisted reproduction techniques.

Mayonnaise

Mayonnaise (, , also US: ), informally mayo (), is a thick cold condiment or dressing usually used in sandwiches and composed salads or on chips. It is a stable emulsion of oil, egg yolk, and acid, either vinegar or lemon juice. There are many variants using additional flavorings. The proteins and lecithin in the egg yolk serve as emulsifiers in mayonnaise (and hollandaise sauce). The color of mayonnaise varies from near-white to pale yellow, and its texture from a light cream to a thick gel. It is also a base in sauces such as Tartar sauce.

Commercial egg-free varieties are made for vegans and others who avoid chicken eggs or dietary cholesterol.

Oviparity

Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, and the monotremes.

In traditional usage, most insects, molluscs, and arachnids are also described as oviparous.

Phoenix (mythology)

In Greek mythology, a phoenix (; Ancient Greek: φοῖνιξ, phoînix) is a long-lived bird that cyclically regenerates or is otherwise born again.

Associated with the Sun, a phoenix obtains new life by arising from the ashes of its predecessor. According to some sources, the phoenix dies in a show of flames and combustion, although there are other sources that claim that the legendary bird dies and simply decomposes before being born again. There are different traditions concerning the lifespan of the phoenix, but by most accounts the phoenix lived for 500 years before rebirth. Herodotus, Lucan, Pliny the Elder, Pope Clement I, Lactantius, Ovid, and Isidore of Seville are among those who have contributed to the retelling and transmission of the phoenix motif.

In ancient Greece and Rome, the phoenix was associated with Phoenicia, (modern Lebanon), a civilization famous for its production of purple dye from conch shells.

In the historical record, the phoenix "could symbolize renewal in general as well as the sun, time, the Empire, metempsychosis, consecration, resurrection, life in the heavenly Paradise, Christ, Mary, virginity, the exceptional man, and certain aspects of Christian life".

Yolk

Among animals which produce eggs, the yolk (also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some kinds of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fishes and invertebrates, the yolk material is not in a special organ, but inside the ovum.

As stored food, yolks are often rich in vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in controlling the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization.

The yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm. The food material and associated control structures are supplied during oogenesis. Some of the material is stored more or less in the form in which the maternal body supplied it, partly as processed by dedicated non-germ tissues in the egg, while part of the biosynthetic processing into its final form happens in the oocyte itself.Apart from animals, other organisms, like algae, specially in the oogamous, can also accumulate resources in their female gametes. In gymnosperms, the remains of the female gametophyte serve also as food supply, and in flowering plants, the endosperm.

Zygote

A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual. In multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.

Oscar Hertwig and Richard Hertwig made some of the first discoveries on animal zygote formation.

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