What is Egg (biology)?
: [[File:Vogeleier.jpg|Size of bird eggs varies roughly according to the size of the adult bird. The [[ostrich]] lays a 1.5 [[kg]] egg (right). The other eggs shown are from a [[chicken]] (lower left) and [[quail]] (top left).|thumb|right]] An '''egg''' results from [[fertilization]] of an [[ovum]]. The egg is a container for the [[zygote]]. It protects the zygote, and feeds the [[embryo]]. The animal [[embryo]] develops until it can survive on its own, at which point the egg hatches. Most [[vertebrates]], [[arthropod]]s and [[molluscs]] lay eggs outside the mother's body. They are always in some kind of container, a shell or covering. [[Reptiles]], [[birds]] and [[monotremes]] lay [[cleidoic egg]]s: [[bird]] eggs are an example. These are a special kind of egg with a good supply of food and water. They have an outside covering which lets through gases so that [[carbon dioxide]] can get out, and [[oxygen]] can get in. [[Fish]], [[amphibia]], [[insects]] and [[arachnids]] lay simpler eggs in greater numbers, but with much less protection and nourishment. == Live birth == Some animals, mostly [[mammal]]s, keep the [[zygote]] inside their body where the [[embryo]] grows until it is ready to be born. These are also cleidoic eggs, but all the development is done inside the mother's body, as with humans. Most mammals, and a number of [[marine biology|marine]] reptiles, give live birth. Also, there are some [[invertebrates]], such as [[scorpion]]s, where the eggs develops inside the mother. They also give birth to live young. == Largest egg == The [[ostrich]] has the largest eggs of all living animals. Eggs, mostly bird eggs, are often eaten as [[food]]. The shell of an egg is a hard calcareous (CaCO<sub>3</sub>) material. The shell of an ostrich egg can support the weight of a fully grown [[human]]. <gallery> Image:Tortoise-Hatchling.jpg|A baby [[tortoise]] hatches from a [[reptile]] egg. Image:Chicks hatching USDA95c1973.jpg|[[Bird]] eggs with hatched chick. Image:Frog in frogspawn.jpg|A [[frog]] with frog