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An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, mollusks, fish, and two types of mammals (the echidna and the platypus) an egg (Latin, ovum) is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing embryo can survive on its own. Oviparous animals are animals that lay eggs, with little or no other development within the mother. The study or collecting of eggs, particularly bird eggs, is called oology. Reptile eggs, bird eggs, and monotreme eggs, which are laid out of water, are surrounded by a protective shell, either flexible or inflexible. The special membranes that support these eggs are traits of all amniotes, including mammals. Eggs laid on land or in nests are usually kept within a favourable temperature range (warm) while the embryo grows. When the embryo is adequately developed it breaks out of the egg’s shell. This breaking out is known as "hatch-ing". Baby animals which have just hatched are "hatch-lings", though standard names for babies of particular species continue to apply, such as chick for a baby chicken. Some embryos have a temporary egg tooth with which to crack, pip, or break the eggshell or covering. The 1.5 kg (3.3 lb) ostrich egg is the largest egg currently known, though the extinct Aepyornis and some dinosaurs have had larger eggs. The Bee Hummingbird produces the smallest known bird egg, which weighs half of a gram. 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 and others (click on image for key) 
Guillemot eggs
Turtle eggs in a nest dug by a female common snapping turtle (Chelydra serpentina) 
A frog amongst frogspawn From Wikipedia, the free encyclopedia : Wholesale Eggs |
