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, such as scorpions and most mammals, do not. 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.e. breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering. The largest recorded egg is from a whale shark, and was 30 cm × 14 cm × 9 cm (11.8 in × 5.5 in × 3.5 in) in size. Whale shark eggs typically hatch within the mother.
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