From Big Medical Encyclopedia

POSTEMBRYONAL DEVELOPMENT (Greek post after + embryon a germ, a fruit) — development of an organism after the birth (or hatching of egg).

At the person and mammals the concept «postembryonal development» practically matches the concept «post-natal development»; preceding the birth pre-natal (prenatal, antenatal) development is subdivided on embryonal and fetal (fetalis) by the periods. The concept «postembryonal development» is inapplicable to the elementary and those lowest backboneless animals, in a life cycle to-rykh there is no stage of egg or live birth. In botany this concept is applied only in relation to spermaphytes, a life cycle to-rykh is subdivided into the embryonal period (formation of seedbuds and seeds) and P. of river including vegetative and generative phases.

At the majority of backboneless animals the entire period of P. of river is subdivided on one or more larval stages and final puberal (adult, imaginal, definitivny) a stage. At the same time larvae (see) can differ markedly from an adult organism (imago) on morphology, physiology and even on the habitat. E.g., at many insects of a larva have a worm-shaped body, are deprived of wings and extremities, live in the soil or water, and an imago — the typical winged, difficult differentiated forms. In the presence of two and more larval stages (larval age) at insects with hemimetaboly with each molt of a larva not only increase in sizes, but also more and more approach on the structure an adult form; at insects with full transformation the structure of larvae of different age changes a little — they only grow, and sharp transition to imaginal type of a structure is made in a stage of a doll and is followed by almost full reorganization of most bodies, i.e. metamorphosis (see).

Among vertebrate animals there is an accurate division of P. of river on larval and adult stages takes place only at fishes (except viviparous) and amphibians, especially tailless (stages a tadpole — a frog). At mammals newborns already have the main lines of a structure inherent to adult organisms; in the period of P. of river there is first of all a growth of an organism which is combined with maturing of separate bodies and systems, underdeveloped at newborns (see. Ontogenesis , Organogenesis ).

At the person rubles and the pubertal period are especially intensive early P. Thus separate parts of a body grow unequally that leads to essential changes of its proportions (the child — the teenager — the adult). In the pubertal period there is a puberty of an organism, development of secondary sexual characteristics and essential reorganization of physiological and mental processes.

Average rates of P. of river are species-specific and rather constant, though are subject to the known variability that depends both on hereditary factors, and on conditions of the environment. A special case of such variability at the person — acceleration (see), or P.'s acceleration by the river, including the increase in growth and earlier puberty observed within the last decades in the different countries of the world. The reasons of acceleration are completely not opened, but the role in it social factors in the conditions of a scientific and technological revolution — change of the mode of work, rest and food, family way (e.g., malodetny families), early «socialization» of teenagers owing to universal distribution of mass media, etc. is undoubted.

From P.'s disturbances by the river of the person the large number (several cells) of forms of the endogenous (hereditary) anomalies mentioning the different parties of its somatic and mental development is known (see. Hereditary diseases ). Alimentary insufficiency, defects of physical and intellectual training, the postponed diseases, injuries, etc.

See also can be the reasons of deviations in P. by river Life cycle .

Bibliography: Zussman M. Developmental biology, the lane with English, M., 1977; The Teratology of the person, under the editorship of G. I. Lazyuk, M., 1979; Harrison J., etc. Human biology, the lane with English, M., 1968.

V. I. Ivanov.