 Embryology is the branch of biology that studies the prenatal development of gametes sex cells fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology. Embryology is a long history. Aristotle proposed the currently accepted theory of epigenesis that organisms develop from seed or egg in a sequence of steps. The alternative theory, preformationism, that organisms develop from pre-existing miniature versions of themselves, however it held sway until the 18th century. Modern embryology developed from the work of von Bader, though accurate observations had been made in Italy by anatomists such as Aldrath and Leonardo VIII Vinci in the Renaissance. In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves—see, embryological origins of the mouth and anus. If in the blastula the first port blastoport becomes the mouth of the animal, it is a protostome. If the first port becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and mollusks, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrole. The gastrole with its blastoport soon develops three distinct layers of cells—the germ layers from which all the bodily organs and tissues then develop. The innermost layer, or endoterm, gives the rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites. The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system. The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales. Membrials in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.