An egg is the organic vessel containing the zygote in which an animal embryo develops until it can survive on vertebrates, and mollusks lay eggs, although some, such as scorpions and most mammals, do not.
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. Most arthropods, vertebrates, and mollusks lay eggs, although some, such as scorpions and most mammals, do not.