Egg

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Eggs of various birds, a reptile, various cartilaginous fish, a cuttlefish and various butterflies and moths. (Click on image for key) Oeufs002b.jpg
Eggs of various birds, a reptile, various cartilaginous fish, a cuttlefish and various butterflies and moths. (Click on image for key)
Diagram of a fertilized chicken egg in its ninth day. Membranes: allantois, chorion, amnion, and vitellus/yolk. Chicken egg diagram.svg
Diagram of a fertilized chicken egg in its ninth day. Membranes: allantois, chorion, amnion, and vitellus/yolk.
Six commercial chicken eggs -- view from the top against a white background Six eggs views from the top on a white background.jpg
Six commercial chicken eggs view from the top against a white background

An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the animal hatches.

Contents

Most arthropods, vertebrates (excluding live-bearing mammals), and mollusks lay eggs, although some, such as scorpions, do not.

Reptile eggs, bird eggs, and monotreme eggs are laid out of water and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches, i.e., breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering.

The largest recorded egg is from a 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] :130 though the extinct elephant bird and some non-avian dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg, which measures between 6.35–11.4 millimetres (0.250–0.449 in) long and weighs half of a gram (around 0.02 oz). [2] :132 Some eggs laid by reptiles and most fish, amphibians, insects, and other invertebrates can be even smaller.

Reproductive structures similar to the egg in other kingdoms are termed "spores", or in spermatophytes "seeds", or in gametophytes "egg cells".

Eggs of different animal groups

Several major groups of animals typically have readily distinguishable eggs.

Overview of eggs from various animals
Class Types of eggsDevelopment
Jawless fish Mesolecithal eggs, especially large in hagfish [3] Larval stage in lampreys, direct development in hagfish. [4] [5] [ page needed ]
Cartilaginous fish Macrolecithal eggs with egg capsule [3] Direct development, viviparity in some species [6] [ page needed ]
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.jpg
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).jpg
Diagram of a fish egg: A. vitelline membrane B. chorion C. yolk D. oil globule E. perivitelline space F. embryo
Salmon fry hatching. The larva has grown around the remains of the yolk and the remains of the soft, transparent egg are discarded. Salmonlarvakils.jpg
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.

Moor frog eggs utilize glycans to form a hydrophilic jelly coat that protects the egg RanaArvalisMatingPlusSpawn.jpg
Moor frog eggs utilize glycans to form a hydrophilic jelly coat that protects the egg

The eggs of fish and amphibians are jellylike. Cartilaginous fish (sharks, skates, rays, chimaeras) eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development. Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina .

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.

Colours

Guillemot eggs Uria aalge MHNT Box Rouzic.jpg
Guillemot eggs

The default colour of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce coloured eggs. The colour comes from pigments deposited on top of the calcium carbonate base; biliverdin and its zinc chelate, and bilirubin, give a green or blue ground colour, while protoporphyrin IX produces reds and browns as a ground colour or as spotting. [10]

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 coloured eggs, even if there is no need of cryptic colors. However, some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid-state lubricant. [11] 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. [12]

For the same reason, later eggs in a clutch are more spotted than early ones as the female's store of calcium is depleted.

The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex-determining W chromosome (female birds are WZ, males ZZ).

It used to be thought that color was applied to the shell immediately before laying, but subsequent research shows that coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral.

In species such as the common guillemot, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed.

Yolks of birds' eggs are yellow from carotenoids, it is affected by their living conditions and diet. [10]

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. [13]

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. [14]

Shape

A 3D model of an egg Oeuf.stl
A 3D 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.[ citation needed ]. One hypothesis is that long, pointy eggs are an incidental consequence of having a streamlined body typical of birds with strong flying abilities; flight narrows the oviduct, which changes the type of egg a bird can lay. [15] [16]

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. [17]

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

Amniote eggs and embryos

Turtle eggs in a nest dug by a female common snapping turtle (Chelydra serpentina) Snapping turtle eggs md.jpg
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. [18]

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

Orange-peel doris (Acanthodoris lutea), a nudibranch, in tide pool laying eggs Editing Image-Acanthodoris lutea laying eggs 2.jpg
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. [19]

Formation

The cycle of the egg's formation is started by the gamete ovum being released (ovulated) and egg formation being started. The finished egg is then ovipositioned and eventual egg incubation can start.

Scientific classifications

Scientists often classify animal reproduction according to the degree of development that occurs before the new individuals are expelled from the adult body, and by the yolk which the egg provides to nourish the embryo.

Egg size and yolk

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

Microlecithal eggs from the roundworm Toxocara Toxocara embryonated eggs.jpg
Microlecithal eggs from the roundworm Toxocara
Microlecithal eggs from the flatworm Paragonimus westermani Paragonimus westermani 01.jpg
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. [20]

Microlecithal eggs require minimal yolk mass. Such eggs are found in flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods. [21] 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 is mesolecithal. Frogspawn closeup.jpg
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] [21] 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

A baby tortoise begins to emerge "fully developed" from its macrolecithal egg. Tortoise-Hatchling.jpg
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] [22]

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 (3.5 in) in diameter, and the young go through full development while in the uterus, living on the copious yolk. [23]

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: [24]

The term hemotrophic derives from the Latin for blood-feeding, contrasted with histotrophic for tissue-feeding. [29]

Human use

Food

Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia. Popular choices for egg consumption are chicken, duck, roe, and caviar, but by a wide margin the egg most often humanly consumed is the chicken egg, typically unfertilized.

Eggs and Kashrut

According to the Kashrut, that is the set of Jewish dietary laws, kosher food may be consumed according to halakha (Jewish law). 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. [30]

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

Chocolate Easter eggs hidden as part of an egg hunt Easter Egg Hunt (5623253840).jpg
Chocolate Easter eggs hidden as part of an egg hunt

Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths. [31] Egg decoration is a common practice in many cultures worldwide. Christians view Easter eggs as symbolic of the resurrection of Jesus Christ. [32] A popular Easter tradition in some parts of the world is the decoration of hard-boiled eggs (usually by dyeing, but often by hand-painting or spray-painting). Adults often hide the eggs for children to find, an activity known as an Easter egg hunt. A similar tradition of egg painting exists in areas of the world influenced by the culture of Persia. Before the spring equinox in the Persian New Year tradition (called Norouz), each family member decorates a hard-boiled egg and sets them together in a bowl. The tradition of a dancing egg is held during the feast of Corpus Christi in 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. [33]

Although a food item, raw eggs are sometimes thrown at houses, cars, or people. This act, known commonly as "egging" in the various English-speaking countries, is a minor form of vandalism and, therefore, usually a criminal offense and is capable of damaging property (egg whites can degrade certain types of vehicle paint) as well as potentially causing serious eye injury. On Halloween, for example, trick or treaters have been known to throw eggs (and sometimes flour) at property or people from whom they received nothing.[ citation needed ] Eggs are also often thrown in protests, as they are inexpensive and nonlethal, yet very messy when broken. [34]

Collecting

Egg collecting was a popular hobby in some cultures, including European Australians. Traditionally, the embryo would be removed before a collector stored the egg shell. [35]

Collecting eggs of wild birds is now banned by many jurisdictions, as the practice can threaten rare species. In the United Kingdom, the practice is prohibited by the Protection of Birds Act 1954 and Wildlife and Countryside Act 1981. [36] On the other hand, ongoing underground trading is becoming a serious issue. [37]

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, [38] and the collection in Western Australia Museum has been archived in a gallery. [39] 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. [40]

See also

Related Research Articles

<span class="mw-page-title-main">Gestation</span> Period during the carrying of an embryo

Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals. It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth.

<span class="mw-page-title-main">Birth</span> Process of bearing offspring

Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe.

The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will degenerate in the body. Normally, these are paired structures, but in birds and some cartilaginous fishes, one or the other side fails to develop, and only one functional oviduct can be found.

<span class="mw-page-title-main">Egg cell</span> Female reproductive cell in most anisogamous organisms

The egg cell or ovum is the female reproductive cell, or gamete, in most anisogamous organisms. The term is used when the female gamete is not capable of movement (non-motile). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified as oogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere. When fertilized, the oosphere becomes the oospore.

<span class="mw-page-title-main">Amniote</span> Clade of tetrapods including reptiles, birds and mammals

Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibian ancestors during the Carboniferous period and further diverged into two groups, namely the sauropsids and synapsids, an event that marks the appearance of Amniota, according to the definition established under the PhyloCode. This basal divergence within Amniota has been dated by molecular studies at 310–329 Ma or 312–330 Ma, but the presence of Hylonomus at Joggins implies a minimal age of about 317 Ma. A fossilized birth-death process study of early amniotes suggested an age of 322–340 Ma. Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians — by the development of three extraembryonic membranes, thicker and keratinized skin, and costal respiration. Additional unique features are the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys, which are more complex, the presence of an astragalus for better extremity range of motion, the diminished role of skin breathing, and the complete loss of metamorphosis, gills, and lateral lines.

<span class="mw-page-title-main">Viviparity</span> Development of the embryo inside the mother

In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juvenile that is at least metabolically independent. This is opposed to oviparity, where the embryos develop independently outside the mother in eggs until they are developed enough to break out as hatchlings; and ovoviviparity, where the embryos are developed in eggs that remain carried inside the mother's body until the hatchlings emerge from the mother as juveniles, similar to a live birth.

<span class="mw-page-title-main">Ovoviviparity</span> Gestation type

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 inside eggs that remain in the mother's body until they are ready to hatch.

<span class="mw-page-title-main">Yolk</span> Part of an egg which feeds the developing embryo

Among animals which produce eggs, the yolk is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient 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 fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell.

<span class="mw-page-title-main">Egg incubation</span> The process by which certain egg-laying animals hatch their eggs

Egg incubation is the process by which an egg, of oviparous (egg-laying) animals, develops an embryo within the egg, after the egg's formation and ovipositional release. Egg incubation is done under favorable environmental conditions, possibly by brooding and hatching the egg.

<span class="mw-page-title-main">Animal embryonic development</span> Process by which the embryo forms and develops

In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula, or of the blastocyst in mammals.

<span class="mw-page-title-main">Internal fertilization</span> Union of an egg and sperm to form a zygote within the female body

Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For internal fertilization to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract.

<span class="mw-page-title-main">Fish reproduction</span> Reproductive physiology of fishes

Fish reproductive organs include testes and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused. There may also be a range of secondary organs that increase reproductive fitness. The genital papilla is a small, fleshy tube behind the anus in some fishes, from which the sperm or eggs are released; the sex of a fish can often be determined by the shape of its papilla.

<span class="mw-page-title-main">Oviparity</span> Animals that lay their eggs, with little or no other embryonic development within the mother

Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings known as hatchlings with little or no embryonic development within the mother. This is the reproductive method used by most animal species, as opposed to viviparous animals that develop the embryos internally and metabolically dependent on the maternal circulation, until the mother gives birth to live juveniles.

<span class="mw-page-title-main">Eggshell</span> Protective exterior of an egg

An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.

In biology, polyspermy describes the fertilization of an egg by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome—one from the egg and one each from multiple sperm. Usually, the result is an unviable zygote. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. Such a situation is often deleterious to the female: in other words, the male–male competition among sperm spills over to create sexual conflict.

<span class="mw-page-title-main">Bird egg</span> Component of avian reproduction

Bird eggs are laid by the females and range in quantity from one to up to seventeen. Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been fertilized; it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs, which are sometimes called wind-eggs.

<span class="mw-page-title-main">Anamniotes</span> Group of vertebrates consisting of amphibians and fish

The anamniotes are an informal group of craniates comprising all fishes and amphibians, which lay their eggs in aquatic environments. They are distinguished from the amniotes, which can reproduce on dry land either by laying shelled eggs or by carrying fertilized eggs within the female. Older sources, particularly before the 20th century, may refer to anamniotes as "lower vertebrates" and amniotes as "higher vertebrates", based on the antiquated idea of the evolutionary great chain of being.

<span class="mw-page-title-main">Pregnancy in fish</span>

Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after the egg-sperm union. Although the term often refers to placental mammals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g. Consistent with this definition, there are several modes of reproduction in fish, providing different amounts of parental care. In ovoviviparity, there is internal fertilization and the young are born live but there is no placental connection or significant trophic (feeding) interaction; the mother's body maintains gas exchange but the unborn young are nourished by egg yolk. There are two types of viviparity in fish. In histotrophic viviparity, the zygotes develop in the female's oviducts, but she provides no direct nutrition; the embryos survive by eating her eggs or their unborn siblings. In hemotrophic viviparity, the zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta.

<span class="mw-page-title-main">Modes of reproduction</span>

Animals make use of a variety of modes of reproduction to produce their young. Traditionally this variety was classified into three modes, oviparity, viviparity, and ovoviviparity.

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