An egg tooth is a temporary, sharp projection present on the bill or snout of an oviparous animal at hatching. It allows the hatchling to penetrate the eggshell from inside and break free. Birds, reptiles, and monotremes possess egg teeth as hatchlings. Similar structures exist in eleutherodactyl frogs (the group known to bypass the tadpole stage), several groups of insects (the structure known as egg burster) and spiders.
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Egg tooth in the context of Egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell – a zygote. Within the vessel, an embryo is incubated until it has become an animal fetus that can survive on its own, at which point the animal hatches. Reproductive structures similar to the egg in other kingdoms are termed "spores", or in spermatophytes "seeds", or in gametophytes "egg cells".
Most arthropods, vertebrates (excluding live-bearing mammals), and mollusks lay eggs, although some, such as scorpions, 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.