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In most birds and reptiles, an egg (Latin ovum) is the zygote, resulting from fertilization of the ovum. It nourishes and protects the embryo. Oviparous animals are animals that lay eggs, with little or no other development within the mother. This is the reproductive method of many fish, amphibians and reptiles, all birds, the monotremes, and most insects and arachnids. Reptile eggs, bird eggs, and monotreme eggs, which are laid out of water, are surrounded by a protective shell, either flexible or inflexible. The 1.5 kg ostrich egg contains the largest existing single cell currently known, though the extinct Aepyornis and some dinosaurs had larger eggs. The bee hummingbird produces the smallest known bird egg, which weighs half a gram. The eggs laid by some reptiles and most fish are even smaller, and those of insects and other invertebrates are much smaller still. The study or collecting of eggs, in particular bird eggs, is called oology. Image:Tortoise-Hatchling.jpg|A baby tortoise emerges from a reptile egg. Image:Emperor Gum Moth eggs2.jpg|Insect eggs, in this case those of the Emperor Gum Moth, are often laid on the underside of leaves. Image:Clupeaharenguskils2.jpg|Fish eggs, such as these herring eggs are often transparent and are fertilized after laying Image:Egg125o.gif|An average Whooping Crane egg is 102 mm long, and weighs 208 grams
Bird eggs Usually after fertilization, the bird egg is laid by the female and is incubated for a time that varies according to the species; then 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, and it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs. Shell structure
Shape
Predation There are numerous animals that feed on eggs. Principal predators of the Black Oystercatcher's eggs, for example, 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. Other mammals, like humans, also eat bird eggs. The egg-eating snakes (genera Dasypeltis and Elachistodon) specialize in eating eggs. Brood parasitism also 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, most famously the Common Cuckoo. Fish eggs
Mammal eggs
Reptile eggs Reptile eggs are rubbery and are always initially white. Often the sex of the developing embryo is determined by the temperature of the surroundings (cool temperatures produce males while warm temperatures produce female offsprings). Not all reptiles lay eggs; some are viviparous. Amphibian eggs In many ways similar to fish eggs, amphibian eggs are jellylike and are fertilised externally. They also do not have a shell and therefore need to be laid in water. See also | ||||||||||||||||
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