Egg
Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows.
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).
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.
In certain instances, the physically most developed offspring will devour its smaller siblings for further nutrition while still within the mother's body.
Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina.
[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.
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).
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.
For example, principal predators of the black oystercatcher's eggs include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes.
Among mammals, early extinct species laid eggs, as do platypuses and echidnas (spiny anteaters).
Marsupial and placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes.
The eggs of marsupials are likewise macrolecithal, but rather small, and develop inside the body of the female, but do not form a placenta.
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.
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.
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.
In sponges and cnidarians, the dividing eggs develop directly into a simple larva, rather like a morula with cilia.
Such eggs are found in flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods.
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.
[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.
[29] Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by people for millennia.
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.
Eggs are an important symbol in folklore and mythology, often representing life and rebirth, healing and protection, and sometimes featuring in creation myths.
[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).
Before the spring equinox in the Persian New Year tradition (called Norouz), each family member decorates a hard-boiled egg and sets them together in a bowl.
The tradition of a dancing egg is held during the feast of Corpus Christi in Catalan cities since the 16th century.
It consists of an emptied egg, positioned over the water jet from a fountain, which starts turning without falling.
[33][unreliable source] 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.
[38] 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.
