Phylembryogeny

evolutionary change that occurs in the course of the development of an organism. The term “phylembryogeny” was introduced in 1910 by A. N. Severtsov.

The theory of phylembryogeny rests mainly on the concept of the predominance of ontogenetic change over phylogenetic (evolutionary) change. If the course of ontogeny did not change, organisms would not differ from their ancestors. As a result of phylembryogeny, ontogenetic change may affect an entire organism or individual organs, tissues, and cells. Changes may occur in the adult stage or in any of the intermediate stages of an organism’s development.

The most important modes of phylembryogeny are anaboly (extension of the terminal stages of development), deviation (change in the middle stages), and archallaxis (change in the primordial rudiments). Thus, the modes of phylembryogeny differ in their time of development and in the character of their evolutionary transformations. Development is said to be progressive when the organism’s structure and functions become more complex. It is considered regressive when the structure and functions become simpler as a result of adaptation to less diverse living conditions, for example, in cases of parasitism.