In the first half of the 19th century, studying the embryonic development of various vertebrates, the domestic scientist Karl Maksimovich Baer established the law of germinal similarity. This law states that in the early stages of embryonic development, organisms are similar to the corresponding stages of development of ancestral and related forms. Thus, type traits are formed in embryos, for example, chordates, earlier than special class traits. At the first stage, all embryos have gill slits, a chord, and a two-chamber heart. At the second stage, features characteristic of each class appear, and only at the third stage are signs of orders, genera, and species formed. Embryological methods for studying evolution have made it possible to establish connections between the individual development of organisms – ontogenesis and the historical development of systematic groups – phylogenesis. So, in the middle of the XIX century. German scientist Fritz Müller, after conducting a series of observations on the development of crustaceans, suggested that their ancestors might look like larvae of modern forms. Later, another German biologist, Ernst Haeckel, based on the discoveries made by Müller, compiled the first phylogenetic tree of the animal world and formulated a biogenetic law : ontogenesis of an individual is a short and rapid repetition of its phylogenesis. According to the Haeckel-Müller biogenetic law, in the individual development of any species in the early stages, features of ancient ancestral forms are found, and in later stages – evolutionarily young. For example, the tadpole frogues. repeats the stage of fish in the development of amphibians, and the butterfly caterpillar – the vermiform stage of the ancestors of insects.