Source:- google.com.pk
An egg is the organic vessel containing the
zygote in which an animal embryo develops until it can survive on its own, at
which point the animal hatches. An egg results from fertilization of an ovum.
Most arthropods, vertebrates, and mollusks lay eggs, although some do not, such
as scorpions and most mammals.
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 favorable
temperature range (warm) 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 with which to crack, pip, or break the eggshell or
covering.
The largest recorded egg is from a whale shark,
and was 30 × 14 × 9 cm (11.8 × 5.5 × 3.5 in) in size; whale shark eggs normally
hatch within the mother. At 1.5 kg (3.3 lb) and up to 17.8 cm × 14 cm (7.0 in ×
5.5 in), the ostrich egg is the largest egg of any living bird,[2] though the
extinct elephant bird and some dinosaurs laid larger eggs. The bee hummingbird
produces the smallest known bird egg, which weighs half of a gram (around 0.2
oz). The eggs laid by some reptiles and most fish can be even smaller, and those
of insects and other invertebrates can be much smaller still.
Bird Eggs
Bird eggs are laid by females and incubated for a
time that varies according to the species; a single young hatches from each
egg. Average clutch sizes range from one (as in condors) to about 17 (the Grey
Partridge). Some birds lay eggs even when not fertilized (e.g. hens); it is not
uncommon for pet owners to find their lone bird nesting on a clutch of
unfertilized eggs, which are sometimes called wind-eggs.
Colors
Guillemot Eggs
The default color of vertebrate eggs is the white
of the calcium carbonate from which the shells are made, but some birds, mainly
passerines, produce colored eggs. The pigment biliverdin and its zinc chelate
give a green or blue ground color, and protoporphyrin produces reds and browns
as a ground color or as spotting.
Non-passerines typically have white eggs, except
in some ground-nesting groups such as the Charadriiformes, sandgrouse and
nightjars, where camouflage is necessary, and some parasitic cuckoos which have
to match the passerine host's egg. Most passerines, in contrast, lay colored
eggs, even if there is no need of cryptic colors.
However some have suggested that the
protoporphyrin markings on passerine eggs actually act to reduce brittleness by
acting as a solid state lubricant.[10] If there is insufficient calcium
available in the local soil, the egg shell may be thin, especially in a circle
around the broad end. Protoporphyrin speckling compensates for this, and
increases inversely to the amount of calcium in the soil.
For the same reason, later eggs in a clutch are
more spotted than early ones as the female's store of calcium is depleted.
The color of individual eggs is also genetically
influenced, and appears to be inherited through the mother only, suggesting
that the gene responsible for pigmentation is on the sex determining W
chromosome (female birds are WZ, males ZZ).
It used to be thought that color was applied to
the shell immediately before laying, but this research shows that coloration is
an integral part of the development of the shell, with the same protein
responsible for depositing calcium carbonate, or protoporphyrins when there is
a lack of that mineral.
In species such as the Common Guillemot, which
nest in large groups,each female's eggs have very different markings, making it
easier for females to identify their own eggs on the crowded cliff ledges on
which they breed.
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.
Human Use
Food
Eggs laid by many different species, including
birds, reptiles, amphibians, and fish, have probably been eaten by mankind for
millennia. Popular choices for egg consumption are chicken, duck, roe, and
caviar, but by a wide margin the egg most often humanly consumed is the chicken
egg, typically unfertilized.
Eggs and Kashrut
According to the Kashrut, that is the set of
Jewish dietary laws, kosher food may be consumed according to halakha (Jewish
law). Kosher meat and milk (or derivatives) cannot be mixed (Deuteronomy 14:21)
or stored together. Eggs are considered pareve (neither meat nor dairy) despite
being an animal product and can be mixed with either milk or kosher meat.
Mayonnaise, for instance, is usually marked "pareve" despite by
definition containing egg.
Culture
A popular Easter tradition in some parts of the
world is the decoration of hard-boiled eggs (usually by dyeing, but often by
spray-painting). Adults often hide the eggs for children to find, an activity
known as an Easter egg hunt. A similar tradition of egg painting exists in
areas of the world influenced by the culture of Persia. Before the spring
equinox in the Persian New Year tradition (called Norouz), each family member
decorates a hard-boiled egg and sets them together in a bowl. The tradition of
a dancing egg is held during the feast of Corpus Christi in Barcelona and other
Catalan cities since the 16th century. It consists of an emptied egg,
positioned over the water jet from a fountain, which starts turning without
falling.[25] Although a food item, eggs are sometimes thrown at houses, cars,
or people. This act, known commonly as "egging" in the various
English-speaking countries, is a minor form of vandalism and, therefore,
usually a criminal offense and is capable of damaging property (egg whites can
degrade certain types of vehicle paint) as well as causing serious eye
injury.[81] On Halloween, for example, trick or treaters have been known to
throw eggs (and sometimes flour) at property or people from whom they received
nothing. Eggs are also often thrown in protests, as they are inexpensive and
nonlethal, yet very messy when broken.
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