Animal development refers to the process by which a fertilized egg transforms into a fully formed organism. This process involves several stages, including cleavage, where the egg divides into smaller cells, and gastrulation, where these cells reorganize to form different layers that will develop into various tissues and organs. During development, animals undergo specific changes that are guided by their genetic information. This includes the formation of structures like limbs and organs, as well as the differentiation of cells into specialized types. The study of animal development helps scientists understand growth, regeneration, and evolutionary relationships among species.