Among animals which produce

egg 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 a ...

s, the yolk (; also known as the vitellus) is the

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

-bearing portion of the egg whose primary function is to supply

food Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is inge ...

for the development of the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

. Some types 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 A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

of a

parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

) or because the embryo develops in the parent's body, which supplies the food, usually through a

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...

. Reproductive systems in which the mother's body supplies the embryo directly are said to be

matrotrophic Matrotrophy is a form of maternal care during organism development, associated with live birth (viviparity), in which the embryo of an animal or flowering plant is supplied with additional nutrition from the mother (e.g. through a placenta). This ...

; those in which the embryo is supplied by yolk are said to be

lecithotrophic 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, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...

. In many species, such as all

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

s, and most

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s and

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s, the yolk takes the form of a special storage organ constructed in the

reproductive tract The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical sex organs, organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pherom ...

of the mother. In many other animals, especially very small species such as some

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

and

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, the yolk material is not in a special organ, but inside the

egg cell The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...

. As stored food, yolks are often rich in

vitamin A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an Nutrient#Essential nutrients, essential micronutrient that an organism needs in small quantities for the proper functioning of its ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...

s and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

s. The proteins function partly as food in their own right, and partly in regulating 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 Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

. The yolk is not living

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

material like

protoplasm Protoplasm (; ) is the living part of a cell that is surrounded by a plasma membrane. It is a mixture of small molecules such as ions, monosaccharides, amino acid, and macromolecules such as proteins, polysaccharides, lipids, etc. In some defini ...

, but largely passive material, that is to say

deutoplasm The deutoplasm comprises the food particles stored in the cytoplasm of an ovum or a cell, as distinguished from protoplasm, the yolk substance. Generally, the deutoplasm accumulates about the nucleus and is heavier than the surrounding cytoplasm. I ...

. The food material and associated control structures are supplied during

oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...

. 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 Biosynthesis is a multi-step, enzyme-Catalysis, catalyzed process where substrate (chemistry), substrates are converted into more complex Product (chemistry), products in living organisms. In biosynthesis, simple Chemical compound, compounds are mo ...

processing into its final form happens in the

oocyte An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...

itself. Apart from animals, other organisms, like

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

, especially in the

oogamous Oogamy is an extreme form of anisogamy where the gametes differ in both size and form. In oogamy the large female gamete (also known as ovum) is immobile, while the small male gamete (also known as sperm) is mobile. Oogamy is a common form of an ...

, can also accumulate resources in their female gametes. In

gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, ''Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμνό� ...

s, the remains of the female

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

serve also as food supply, and in

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

s, the

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the embryo and ...

Chicken egg yolk

In avian eggs, the yolk usually is a hue of yellow in color. It is spherical and is suspended in the

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 arou ...

(known alternatively as

albumen 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 arou ...

or glair/glaire) by one or two spiral bands of tissue called the

chalaza The chalaza (; from Greek "hailstone"; plural ''chalazas'' or ''chalazae'', ) is a structure inside bird eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure. In animals In the eggs of most birds (n ...

e. The yolk mass, together with the ovum ''proper'' (after fertilization, the

) are enclosed by the

vitelline membrane The vitelline membrane or vitelline envelope is a structure surrounding the outer surface of the plasma membrane of an ovum (the oolemma) or, in some animals (e.g., birds), the extracellular yolk and the oolemma. It is composed mostly of protein f ...

, whose structure is different from a

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. The yolk is mostly extracellular to the

oolemma The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

, being not accumulated inside the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

of the egg cell (as occurs in

frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...

s), contrary to the claim that the avian ovum (in strict sense) and its yolk are a single giant cell. After the fertilization, the cleavage of the embryo leads to the formation of the

germinal disc The blastodisc, also called the germinal disc, is the embryo-forming part on the yolk of the egg of an animal that undergoes discoidal meroblastic cleavage. Discoidal cleavage occurs in those animals with a large proportion of yolk in their eggs, ...

. As

, the

chicken egg Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

yolk is a major source of vitamins and minerals. It contains all of the egg's

fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers spec ...

and

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...

, and nearly half of the

. If left intact when an egg is fried, the yellow yolk surrounded by a flat blob of egg white creates a distinctive "sunny-side up" form. Mixing the two components together before cooking results in a yellow (from pale yellow to almost orange, depending on the breed of hen) mass, as in

omelet In cuisine, an omelette (also spelled omelet) is a dish made from beaten eggs, fried with butter or oil in a frying pan (without stirring as in scrambled egg). It is quite common for the omelette to be folded around fillings such as chives, ve ...

s and

scrambled eggs Scrambled eggs is a dish made from eggs (usually chicken eggs) stirred, whipped or beaten together while being gently heated, typically with salt, butter, oil and sometimes other ingredients. Preparation Only eggs are necessary to make scrambled ...

Uses

* The developing embryo inside the egg uses the yolk as sustenance. * It is at times separated from the egg white for cooking, and is frequently employed as an

emulsifier An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...

, and is used in

mayonnaise Mayonnaise (; ), colloquially referred to as "mayo" , is a thick, cold, and creamy sauce or dressing commonly used on sandwiches, hamburgers, composed salads, and French fries. It also forms the base for various other sauces, such as tartar ...

custard Custard is a variety of culinary preparations based on sweetened milk, cheese, or cream cooked with egg or egg yolk to thicken it, and sometimes also flour, corn starch, or gelatin. Depending on the recipe, custard may vary in consistency fro ...

hollandaise sauce Hollandaise sauce ( or ; ), also called Dutch sauce, is a mixture of egg yolk, melted butter, and lemon juice (or a white wine or vinegar reduction). It is usually seasoned with salt, and either white pepper or cayenne pepper. It is well kn ...

, ''

crème brûlée ''Crème brûlée'' or ''crème brulée'' (; ), also known as burnt cream or Trinity cream, and virtually identical to the original crema catalana, is a dessert consisting of a rich custard base topped with a layer of hardened caramelized sugar ...

'', ''

avgolemono Avgolemono ( el, αυγολέμονο or literally egg–lemon) is a family of sauces and soups made with egg yolk and lemon juice mixed with broth, heated until they thicken. They are found in Greek, Arab, Sephardic Jewish, Turkish, Balkan an ...

'' and ''

ovos moles Ovos moles de Aveiro (literally, "soft eggs from Aveiro")—sometimes written as ovos-moles de Aveiro—are a local pastry delicacy from Aveiro District, Portugal, made of egg yolks and sugar, and sometimes chocolate. This mixture is then put in ...

''. * It is used in painting as a component of traditional egg-

tempera Tempera (), also known as egg tempera, is a permanent, fast-drying painting medium consisting of colored pigments mixed with a water-soluble binder medium, usually glutinous material such as egg yolk. Tempera also refers to the paintings done ...

. * It is used in the production of egg yolk

agar plate An agar plate is a Petri dish that contains a growth medium solidified with agar, used to culture microorganisms. Sometimes selective compounds are added to influence growth, such as antibiotics. Individual microorganisms placed on the plate wil ...

medium, useful in testing for the presence of ''

Clostridium perfringens ''Clostridium perfringens'' (formerly known as ''C. welchii'', or ''Bacillus welchii'') is a Gram-positive, rod-shaped, anaerobic, spore-forming pathogenic bacterium of the genus '' Clostridium''. ''C. perfringens'' is ever-present in nature an ...

''. * Egg yolk contains an

antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

called antiglobulin (

IgY Immunoglobulin Y (abbreviated as IgY) is a type of immunoglobulin which is the major antibody in bird, reptile, and lungfish blood. It is also found in high concentrations in chicken egg yolk. As with the other immunoglobulins, IgY is a class of ...

). The antibody transfers from the laying hen to the egg yolk by

passive immunity Passive immunity is the transfer of active humoral immunity of ready-made antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially, when hi ...

to protect both embryo and hatchling from microorganism invasion. * Egg yolk can be used to make liqueurs such as

Advocaat Advocaat or advocatenborrel is a traditional Dutch alcoholic beverage made from eggs, sugar, and brandy. The rich and creamy drink has a smooth, custard-like consistency. The typical alcohol content is generally between 14% and 20% ABV. Its co ...

eggnog Eggnog (), historically also known as a milk punch or an egg milk punch when alcoholic beverages are added, is a rich, chilled, sweetened, dairy-based beverage. It is traditionally made with milk, cream, sugar, egg yolks, and whipped egg white ...

. * Egg yolk is used to extract

egg oil Egg oil ( CAS No. 8001–17–0, INCI: egg oil), also known as egg yolk oil or ovum oil, is derived from the yolk of chicken eggs consisting mainly of triglycerides with traces of lecithin, cholesterol, biotin, xanthophylls lutein and zeaxanthin ...

which has various cosmetic, nutritional, and medicinal uses.

Composition of chicken egg yolk

The yolk makes up about 33% of the liquid weight of the egg; it contains about , three times the energy content of the egg white, mostly due to its fat content. All of the fat-soluble vitamins ( A, D, E and K) are found in the egg yolk. Egg yolk is one of the few foods naturally containing vitamin D. The composition (by weight) of the most prevalent fatty acids in egg yolk typically is: * Unsaturated fatty acids: **

Oleic acid Oleic acid is a fatty acid that occurs naturally in various animal and vegetable fats and oils. It is an odorless, colorless oil, although commercial samples may be yellowish. In chemical terms, oleic acid is classified as a monounsaturated omega ...

, 47% **

Linoleic acid Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are cis-trans isomerism, ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt (chem ...

, 16% **

Palmitoleic acid Palmitoleic acid, or (9''Z'')-hexadec-9-enoic acid, is an omega-7 monounsaturated fatty acid (16:1n-7) with the formula CH3(CH2)5CH=CH(CH2)7COOH that is a common constituent of the glycerides of human adipose tissue. It is present in all tissues ...

, 5% **

Linolenic acid Linolenic acid is a type of naturally-occurring fatty acid. It can refer to either of two octadecatrienoic acids (i.e. with an 18-carbon chain and three double bonds, which are found in the '' cis'' configuration), or a mixture of the two. Lino ...

, 2% * Saturated fatty acids: **

Palmitic acid Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms.Gunstone, F. D., John L. Harwood, and Albert J. Dijkstra. The Li ...

, 23% **

Stearic acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek word στέαρ "''stéar''", which means tallow. ...

, 4% **

Myristic acid Myristic acid (IUPAC name: tetradecanoic acid) is a common saturated fatty acid with the molecular formula CH3(CH2)12COOH. Its salts and esters are commonly referred to as myristates or tetradecanoates. It is named after the binomial name for nutm ...

, 1% Egg yolk is a source of

lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and so ar ...

, as well as egg oil, for cosmetic and pharmaceutical applications. Based on weight, egg yolk contains about 9% lecithin. The yellow color is due to

lutein Lutein (;"Lutein"

zeaxanthin Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji ( wolfberries), ...

, which are yellow or orange

carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...

known as

xanthophyll Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek (, "yellow") and (, "l ...

Yolk proteins

The different yolk's proteins have distinct roles. Phosvitins are important in sequestering

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

, and other cations for the developing embryo. Phosvitins are one of the most

phosphorylated In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

(10%) proteins in nature; the high concentration of phosphate groups provides efficient metal-binding sites in clusters. Lipovitellins are involved in lipid and metal storage, and contain a heterogeneous mixture of about 16% (w/w) noncovalently bound lipid, most being phospholipid. Lipovitellin-1 contains two chains, LV1N and LV1C.

Yolk vitamins and minerals

Yolks hold more than 90% of the

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

thiamine Thiamine, also known as thiamin and vitamin B1, is a vitamin, an essential micronutrient, that cannot be made in the body. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosphorylated forms of thi ...

, vitamin B₆,

folate Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and ...

, vitamin B₁₂, and

pantothenic acid Pantothenic acid, also called vitamin B5 is a water-soluble B vitamin and therefore an essential nutrient. All animals require pantothenic acid in order to synthesize coenzyme A (CoA) – essential for fatty acid metabolism – as well as to, i ...

of the egg. In addition, yolks cover all of the fat-soluble vitamins: A, D, E, and K in the egg, as well as all of the

essential fatty acid Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them. Only two fatty acids are known to be essential for humans: alpha-linolenic ac ...

s. A single yolk from a large egg contains roughly 22 mg of calcium, 66 mg of phosphorus, 9.5 micrograms of

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

, and 19 mg of

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

, according to the USDA.U.S. Department of Agriculture, Agricultural Research Service, 2010. USDA National Nutrient Database for Standard Reference, Release 23, Nutrient Data Laboratory Home Page: http://www.ars.usda.gov/nutrientdata

Double-yolk eggs

Double-yolk eggs occur when ovulation occurs too rapidly, or when one yolk becomes joined with another yolk. These eggs may be the result of a young hen's reproductive cycle not yet being synchronized. The average probability of finding this is 1 in 1200. Double-yolked eggs seldom lead to successful hatchlings without human intervention, as the chicks interfere with each other's hatching process and do not survive. Higher-order yolks are rare, although heavier poultry breeds such as the Buff Orpington have been known to lay triple-yolk eggs occasionally. File:Egg and maxi egg 1.jpg, File:Egg and maxi egg 2.jpg, File:Egg with two yellows.jpg,

Yolkless eggs

Eggs without yolks are known as "dwarf" or "wind" eggs, or the archaic term "cock egg". Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully ready. Mature hens rarely lay a yolkless egg, but sometimes a piece of reproductive tissue breaks away and passes down the tube. Such a scrap of tissue may stimulate the egg-producing glands to react as though it were a yolk and wrap it in albumen, membranes, and a shell as it travels through the egg tube. This is usually what causes an egg to contain a small particle of grayish tissue instead of a yolk. Since these eggs contain no yolk, and therefore cannot hatch, they were traditionally believed to have been laid by roosters. This type of egg occurs in many varieties of fowl and has been found in chickens, both standard and bantams,

guineas The guinea (; commonly abbreviated gn., or gns. in plural) was a coin, minted in United Kingdom of Great Britain and Ireland, Great Britain between 1663 and 1814, that contained approximately one-quarter of an ounce of gold. The name came from t ...

, and

coturnix ''Coturnix'' is a genus of five extant species and five to eight known extinct species of Old World quail. These species are distributed throughout Africa, Eurasia, Australia, and formerly New Zealand. An extinct radiation of flightless, insula ...

quail.

Yolk color

The color of an egg yolk is directly influenced by the makeup of the chicken feed. Egg yolk color is generally more yellow when given a feed containing a large component of yellow, fat-soluble pigments, such as the carotenes in dark green plant material, for example

alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, and silage, as w ...

. Although much emphasis is put onto the color of the egg yolk, it does not reliably reflect the nutritional value of an egg. For example, some of the natural pigments that produce a rich yolk color are

s without much nutritional value, rather than the carotenoids that act as

provitamin A provitamin is a substance that may be converted within the body to a vitamin. The term previtamin is a synonym. The term "provitamin" is used when it is desirable to label a substance with little or no vitamin activity, but which can be converted ...

A in the body. Also, a diet rich in vitamin A itself, but without A-provitamins or xanthophylls, can produce practically colourless yolks that are just as nutritious as any richly colored yolks. Yolks, particularly from free-range eggs, can be of a wide range of colors, ranging from nearly white, through yellow and orange, to practically red, or even olive green, depending on the pigments in their feed. Feeding fowl large amounts of

capsicum ''Capsicum'' () is a genus of flowering plants in the Solanum, nightshade family Solanaceae, native to Americas, the Americas, cultivated worldwide for their chili pepper or bell pepper fruit. Etymology and names The generic name may come f ...

peppers, for example, tends to result in red or deep orange yolks. This has nothing to do with adding colors such as

cochineal The cochineal ( , ; ''Dactylopius coccus'') is a scale insect in the suborder Sternorrhyncha, from which the natural dye carmine is derived. A primarily sessility (motility), sessile parasitism, parasite native to tropical and subtropical Sout ...

to eggs in cooking.

In fish

All

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

, some

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...

s and rays have

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...

s at some stage of development, with all oviparous fish retaining the sac after hatching. Lamniform sharks are

ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...

, in that their eggs hatch ''in utero''; in addition to eating unfertilized eggs, unborn sharks participate in intrauterine-

cannibalism Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, b ...

: stronger pups consume their weaker womb-mates.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. . OCLC 156157504.

References

External links

Anatomy of an Egg
from the Exploratorium
Making egg tempera
from the Society of Tempera Painters {{DEFAULTSORT:Egg Yolk Eggs

Chicken egg yolk

Uses

Composition of chicken egg yolk

Yolk proteins

Yolk vitamins and minerals

Double-yolk eggs

Yolkless eggs

Yolk color

In fish

See also

References

External links