Ontogenesis

Ontogenesis, the history of individual or 'ontogenetic' development of a living organism (in distinction from the historical development of its species or 'phylogenesis'), encompassing all the processes of morphological and functional changes experienced by the organism from the beginning of its existence as a fertilized egg until the end of its individual life. Thus, ontogenesis consists of 1) embryonic development of the embryo within the extra-embryonic or embryonic membranes, and 2) post-embryonic development of the more or less free organism. The science studying these processes is called ontogeny (E. Haeckel). Embryonic development is physiologically characterized by the absence of a number of functions inherent in an independent organism that actively procures its own nourishment. The embryo is nourished either from reserves of materials accumulated in the egg itself during its formation ('yolk'), or in special yolk cells (abortive eggs) deposited along with the egg, or in many viviparous forms by substances supplied by the maternal organism in the form of secretions of glandular cells of the ovary or oviducts (uterus), or in the form of nutrient substances transmitted by osmosis from the maternal circulatory system to the embryo's circulatory system through the medium of special organs called placentae.- During post-embryonic development, the organism exhibits considerable independence, usually feeding actively by procuring its own food, although in animals with highly developed care for offspring, the young are sometimes still fed by the mother for some time, for which special adaptations in the form of special glands supplying easily digestible food material often develop (glands in the crop of some birds, especially pigeons, mammary glands of mammals).- With respect to morphogenetic processes, there are significant differences between the two most important periods of O., although the boundary between them cannot be drawn with complete clarity. During embryogenesis, the organism prepares for independent existence, and accordingly during this period all the basic processes of differentiation leading to the formation of organs necessary for independent life occur. The embryonic period quite naturally breaks down into 1) the period of cleavage of the egg, 2) the period of formation of the germ layers (or primary organs), 3) the period of organ formation, histological differentiation, and growth. Embryonic development, particularly the stages of cleavage, formation of germ layers, and initial stages of organogenesis, was often designated as the period of prefunctional development according to the terminology of R. Roux. This viewpoint is methodologically erroneous and does not agree with the facts. It is a reflection of the metaphysical views of contemporary researchers in the field of developmental mechanics, who limit themselves to stating the difference between form and function, ignoring their unity and their interdependence. The factual incorrectness of the concept of the so-called prefunctional period of development is proven by a number of recent works that have investigated changes in functions during ontogenesis, starting from the earliest embryonic stages (Murray, Needham, etc.). Post-embryonic development is characterized, as stated, by the special activity of a free organism, but at the same time the processes of differentiation and growth continue, although more slowly. Post-embryonic development of the organism also breaks down into a number of periods, sometimes clearly demarcated from each other. One can speak of youth, maturity, and old age, with the period of maturity encompassing the time of complete development of the entire organism with the maximum manifestation of its morphological and functional characteristics, while the period of old age is characterized by the onset of certain involutionary processes leading to disruption of optimal coordination of parts and to general weakening of the organism.- Often, an organism hatched from an egg and developing freely differs sharply in its structure (and not only in size), and consequently in its life functions, from the mature organism. In this case, one speaks of a larval period of development. The more pronounced the adaptation of the larval organism to its specific conditions of existence, i.e., the more sharply the larval period is biologically demarcated from the period of maturity (for example, life in different environments, as in amphibians, whose larvae live in water while adult animals live on land), the more extensive the transformations that must occur in the animal's organization during the transition from larval life to adult life. Significant transformations in the internal organization are naturally associated with an interruption of normal vital activity, with a period of relative rest during which more or less profound restructuring of the organism occurs. Such periods are designated as periods of metamorphosis (in many invertebrates, especially insects, among vertebrates in amphibians). The history of individual transformation of the structure and functions of the organism, often associated with changes in the biological environment, is also accompanied by changes in nervous activity, as well as in its highest form—psychic activity. The transition from larval life to life of a mature animal is associated with a complete change in the entire complex of instincts characterizing these periods. In a less pronounced form, however, essentially in exactly the same way, the psyche of every organism changes as it develops individually, as is also vividly expressed in the development of human psyche (with a rather sharp break at the onset of sexual maturity). In the sequence of processes of individual development, i.e., ontogenesis, in different animals, a certain parallelism is observed with the sequence of processes of historical development of their species, i.e., phylogenesis, which gave rise to the formulation of an interesting generalization known as the biogenetic law.