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.