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 ...

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 Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...

for the development of the

embryo An embryo ( ) is the initial stage of development for 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 sp ...

. 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 * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

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 (: placentas or placentae) 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 ...

. 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

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, and most

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s and

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s, 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

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

and

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

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

egg cell The egg cell or ovum (: ova) is the female Reproduction, 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 game ...

. As stored food, yolks are often rich in

vitamin Vitamins are Organic compound, organic molecules (or a set of closely related molecules called vitamer, vitamers) that are essential to an organism in small quantities for proper metabolism, metabolic function. Nutrient#Essential nutrients, ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

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

s and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

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 American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

. The yolk is not living cell material like

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

, but largely passive material, that is to say deutoplasm. The food material and associated control structures are supplied during

oogenesis Oogenesis () or ovogenesis 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 before birth during embryonic devel ...

. 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 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 ger ...

itself. Apart from animals, other organisms, like

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

, especially in the oogamous, can also accumulate resources in their female gametes. In

gymnosperm The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...

s, the remains of the female

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...

serve also as food supply, and in

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

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#Pla ...

Avian 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 a ...

(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 aro ...

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

, whose structure is different from a

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

. .The yolk is part of the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

of the egg, occupying more than 90% of it by the time of fertilization in the form of yolk granules. These are composed of

lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...

proteins 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, re ...

and

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

, with maternal

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...

and

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. It is the main storage form of glucose in the human body. Glycogen functions as one of three regularly used forms ...

surrounding them. After the fertilization, the

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

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 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 specif ...

and

cholesterol Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils. Cholesterol is biosynthesis, biosynthesized by all anima ...

, 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 omelets and

scrambled eggs Scrambled eggs is a dish made from eggs (usually chicken eggs), where the whites and yolks have been stirred, whipped, or beaten together (typically with salt, butter or oil, and sometimes water or milk, or other ingredients), then heated so ...

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. Althou ...

, and is used in

mayonnaise Mayonnaise (), colloquially referred to as "mayo" (), is a thick, creamy sauce with a rich and tangy taste that is commonly used on sandwiches, hamburgers, Salad#Bound salads, bound salads, and French fries. It also forms the base for various o ...

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

hollandaise sauce Hollandaise sauce ( or ; from French meaning "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 ...

, ''

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

'', ''

avgolemono Avgolemono ( or literally egg–lemon) is a family of sauces and soups made with egg (food), egg yolk and lemon juice mixed with broth, heated until they thicken. Avgolemono can be used to thicken soups and stews. Yuvarlakia is a Greek meatball ...

'' and '' ovos moles''. * 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 pigments mixed with a water-soluble binder medium, usually glutinous material such as egg yolk. ''Tempera'' also refers to the paintings done in ...

. * 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 Microbiological culture, culture microorganisms. Sometimes selective compounds are added to influence growth, such as antibiotics. Individual microorganism ...

medium, useful in testing for the presence of ''

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

''. * Egg yolk contains an

antibody An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as pathogenic bacteria, bacteria and viruses, includin ...

called antiglobulin ( IgY). The antibody transfers from the laying hen to the egg yolk by

passive immunity In immunology, 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 arti ...

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

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

. * Egg yolk is used to extract egg oil 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 due to the presence of impurities. In chemical terms, oleic acid is cl ...

, 47% **

Linoleic acid Linoleic acid (LA) is an organic compound with the formula . Both alkene groups () are ''cis''. It is a fatty acid sometimes denoted 18:2 (n−6) or 18:2 ''cis''-9,12. A linoleate is a salt or ester of this acid. Linoleic acid is a polyunsat ...

, 16% ** Palmitoleic acid, 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 ...

, 23% **

Stearic acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a soft waxy solid with the formula . The triglyceride derived from three molecules of stearic acid is called stearin. Stearic acid ...

, 4% **

Myristic acid Myristic acid (IUPAC name: tetradecanoic acid) is a common saturated fatty acid with the molecular formula . Its salts and esters are commonly referred to as myristates or tetradecanoates. The name of the acyl group derived from myristic acid is m ...

, 1% Egg yolk is a source of

lecithin Lecithin ( ; from the Ancient Greek "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 () are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and Fungus, 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: (), meaning "yellow", an ...

Yolk proteins

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

calcium Calcium is a chemical element; it has 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 it ...

iron Iron is a chemical element; it has symbol Fe () 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, forming much of Earth's o ...

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

phosphorylated In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...

(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; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It 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 periodic tabl ...

thiamine Thiamine, also known as thiamin and vitamin B1, is a vitamin – an Nutrient#Micronutrients, essential micronutrient for humans and animals. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosp ...

, 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 (vitamin B5) is a B vitamin and an essential nutrient. All animals need pantothenic acid in order to synthesize coenzyme A (CoA), which is essential for cellular energy production and for the synthesis and degradation of prote ...

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 acids. 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; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

, and 19 mg of

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

, 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. 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, though hens are known to occasionally lay even triple-yolk eggs. 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 Great Britain between 1663 and 1814, that contained approximately one-quarter of an ounce of gold. The name came from the Guinea region in West Africa, from where m ...

, and coturnix 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

carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the ex ...

s in dark green plant material, for example

alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, 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, ...

. 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

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

s 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 converte ...

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 the Americas, cultivated worldwide for their edible fruit, which are generally known as "peppers" or "capsicum". Chili peppers grow on five s ...

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 ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

, some

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s and rays have

yolk sac The yolk sac is a membranous wikt:sac, 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' ...

s at some stage of development, with all

oviparous Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...

fish retaining the sac after hatching. Lamniform sharks are

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

, 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 also well document ...

: 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.

In crustaceans

Pregnant Caridina multidentata in captivity January 2025

The yolk in crustacean eggs is essential for embryonic development, serving as a nutrient reservoir. In decapod crustaceans, the primary yolk precursor protein is apolipocrustacein (apoCr), which differs from the traditional vitellogenins (Vtgs) found in most oviparous animals. ApoCr shares greater structural and evolutionary similarity with insect apolipophorin II/I (apoLp-II/I) and vertebrate apolipoprotein B (apoB), distinguishing it from other members of the large lipid transfer protein (LLTP) superfamily. ApoCr is a large glycolipoprotein, approximately 2,600 amino acids long, with conserved structural domains characteristic of LLTPs. These domains include an N-terminal lipid transfer module, a DUF1081 domain exclusive to apoLp-II/I and apoB, and a von Willebrand factor type D domain at the C-terminal. Additionally, it features a subtilisin-like cleavage site, a trait shared with apoLp-II/I. Evolutionary analyses reveal that apoCr is phylogenetically closer to apoLp-II/I than to Vtg proteins, indicating a distinct lineage for crustacean yolk proteins. In decapods, apoCr is typically expressed in both the ovary and hepatopancreas, supporting its dual roles in lipid metabolism and yolk formation. In some species, gene duplication events have resulted in multiple apoCr variants with tissue-specific functions.

References

External links

Anatomy of an Egg
from the

Exploratorium The Exploratorium is a museum of science museum, science, technology museum, technology, and art museum, arts in San Francisco, California. Founded by physicist and educator Frank Oppenheimer in 1969, the museum was originally located in the ...

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