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Eggs of various birds, a reptile, various cartilaginous fish, a cuttlefish and various butterflies and moths. Diagram of a chicken egg in its 9th day. 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. Whale shark eggs typically hatch within the mother. Reproductive structures similar to the egg in other kingdoms are termed “spores,” or in spermatophytes “seeds,” or in gametophytes “egg cells”. Several major groups of animals typically have readily distinguishable eggs. Larval stage in lampreys, direct development in hagfish.

Larval stage, ovovivipary in some species. Medium-sized mesolecithal eggs in all species. Tadpole stage, direct development in some species. Large macrolecithal eggs, develop independent of water. Large to very large macrolecithal eggs in all species, develop independent of water. The young more or less fully developed, no distinct larval stage.

Macrolecithal eggs in monotremes and marsupials, extreme microlecithal eggs in placental mammals. Young little developed with indistinct larval stage in monotremes and marsupials, direct development in placentals. For fish eggs as food, see Roe. 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. Diagram of a fish egg: A. 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. 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 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. The eggs of fish and amphibians are jellylike. 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. 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.

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. However some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid-state lubricant. 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. 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.

Tiny pores in bird eggshells allow the embryo to breathe. The domestic hen’s egg has around 7000 pores. Some bird eggshells have a coating of vaterite spherules, which is a rare polymorph of calcium carbonate. 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. Brood parasitism occurs in birds when one species lays its eggs in the nest of another.