Embryology is the branch of biology that is concerned with the development of gametes (sex cells), fertilization, and development of embryos and fetuses. It is the science dealing with the formation, development, structure, and functional activities of embryos. Embryology is the study of congenital disorders that occur before birth. Comparative embryology is the branch of embryology that compares and contrasts embryos of different species. It is used to show how all animals are related. Many things are compared (such as whether or not the organism has a notochord or gill arches). Many components go into comparative embryology, and much information about the developmental similarities between species can be taken from its study, from which many conclusions can be drawn.