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ontogeny:see biogenetic lawbiogenetic law,
in biology, a law stating that the earlier stages of embryos of species advanced in the evolutionary process, such as humans, resemble the embryos of ancestral species, such as fish.
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The developmental history of an organism from its origin to maturity. It starts with fertilization and ends with the attainment of an adult state, usually expressed in terms of both maximal body size and sexual maturity. Fertilization is the joining of haploid gametes (a spermatozoon and an ovum, each bearing half the number of chromosomes typical for the species) to form a diploid zygote (with a full chromosome number), a new unicellular living being which will grow through a series of asexual reproductions. The gametes are the link between one generation and the next: the fusion of male and female gametes is the onset of a new ontogenetic cycle. Many organisms die shortly after sexual reproduction, whereas others live longer and generations are overlapped. Species are usually conceived as adults, but in most cases the majority of their representation in the environment is as intermediate ontogenetic stages. See Reproduction (animal)
In unicellular organisms, each asexual reproduction leads to the formation of new individuals, the cells deriving from a first sexually derived individual forming a clone of genetically identical individuals. In multicellular organisms, the products of the asexual reproductions starting with the first division of the zygote remain connected, and the clone they form is a single individual. Clonation of individuals occurs even in humans, when the first results of asexual reproduction of the zygote separate from each other, leading to twin formation.
The ontogeny of a multicellular organism involves segmentation (or cleavage): the zygote divides into two, four, etc., cells which continue to divide. These cells are initially similar to the zygote, although smaller in size. They soon start to differentiate from their ancestors, acquiring special features, and forming specific tissue layers and, eventually, organs. These processes lead to the formation and growth of an embryo. Embryos can develop freely, within egg shells, or within the body of one parent; they can grow directly into juveniles (as in humans) or into larvae (with an indirect development, as in insects).
Juveniles are similar to adults but are smaller in size and not sexually mature. Their ontogeny continues until they reach a maximal size and reproductive ability. Larvae have different morphology, physiology, and ecology from adults; they become juveniles through a metamorphosis (that is, an abrupt change). Usually ontogeny is interrupted at adulthood, but some organisms can grow throughout their life, so that ontogeny ends with their death. See Ecology
the development of an individual organism; the successive morphological, physiological, and biochemical transformations that an organism undergoes in a lifetime.
Ontogeny consists of growth—increase in body size—and differentiation (seeDIFFERENTIATION). The term ontogeny was introduced by E. Haeckel in 1866 in his formulation of the biogenetic law (seeBIOGENETIC LAW). In animals and plants that reproduce sexually, the creation of a new organism takes place during fertilization, and ontogeny begins in the fertilized egg, or zygote. In organisms that reproduce asexually, ontogeny begins with the formation of a new organism by budding from the body of the maternal plant, from a specialized cell, or from specialized structures, such as a rhizome, tuber, or bulb (seeVEGETATIVE REPRODUCTION). During ontogeny, every sexually reproducing organism passes through three successive basic periods of development: prenatal, postnatal, and adult.
Ontogeny is a complex process by which the hereditary information that is contained in each cell of an organism is realized at different stages of the organism’s development. The hereditary program of ontogeny is expressed through the interrelated processes of cell reproduction, growth, and differentiation and is influenced by many factors, including environmental conditions, intercellular and intertissular reactions, and humoral-hormonal and neural regulation. Ontogenetic patterns, causal mechanisms, and factors that affect cell, tissue, and organ differentiation are studied by an interdisciplinary science—developmental biology —which uses not only the traditional approaches of experimental embryology and morphology but also the methods of molecular biology, cytology, and genetics.
Ontogeny and phylogeny—the development of organisms over the course of evolution—are inseparable and mutually dependent aspects of biological development. J. F. Meckel was the first to attempt an evolutionary substantiation of ontogeny. The question of the relationship between phylogeny and ontogeny was raised by C. Darwin and elaborated by other scientists, including F. Müller and Haeckel. All hereditarily determined new traits that appear in evolution arise in ontogeny, but only those that help the organism to better adapt to the environment are retained in the course of natural selection and are transmitted to the following generations, that is, are fixed in evolution. A knowledge of the mechanisms, causes, and factors of ontogeny is the scientific basis for influencing the development of plants, animals, and man. The study of ontogeny is of major importance in plant breeding, livestock raising, and medicine.
In animals. The history of the study of animal ontogeny dates back to the works of the ancient Greek scholars Hippocrates and Aristotle. Beginning in the late 18th century and especially in the 19th and 20th centuries, attention was focused mainly on the prenatal period. Major contributors to the creation of animal embryology included the Russian biologists K. F. Vol’f, Kh. I. Pander, K. M. Baer, E. Metchnikoff, A. O. Kovalevskii, P. P. Ivanov, A. N. Severtsov, and D. P. Filatov; the German zoologists O. Hertwig, R. Hertwig, and Haeckel; and the British em-bryologist F. Balfour.
The prenatal period is usually easily distinguished from the postnatal period by the emergence of the embryo from the egg membrane and from the extraembryonic membranes. In viviparous forms the two periods are separated by the process of birth. The prenatal period consists of three stages: cleavage of the egg, separation of the germ layers, and organogenesis—the formation of individual organs. All multicellular animals are similar in the early stages of development, a phenomenon that is part of the biogenetic law. The juvenile, or initial, stage of postnatal development in animals may occur either by direct development or by metamorphosis. In direct development, the animal that emerges from the egg membrane gradually takes on the adult form without going through a larval stage; this type of development is characteristic of oligochaetes, ctenophorans, leeches, reptiles, birds, mammals, some insects, and most fish. With intrauterine development, which is inherent in placental mammals and man, first the embryo and then the fetus is formed within the mother’s body.
One or more larval stages arises during metamorphosis; this type of development is characteristic of some parasitic flatworms and roundworms, mollusks, most arthropods, some fish, and amphibians. The larvae lead a free mode of life, feed independently, and are specially adapted with larval organs, for example, gills in dragonfly larvae, a yolk sac in fish fry, and gills, a tail, and adhesive organs in tadpoles; these adaptations are very important in larvae but are absent in adult forms. Several transition cases between direct and metamorphic development are recognized, for example, development in orthopterans, hemipterans, and blattarians.
The rate of growth and development of organisms is dependent not only” on hereditary factors but also on nutrition and on temperature, moisture, light, and other environmental factors. The appearance of species traits ends with the onset of sexual maturity, but the development of individual traits continues to the end of ontogeny. In some animals, for example, birds, growth virtually ceases at sexual maturity, while in others, for example, fish, it continues throughout life. The duration of ontogeny varies among different species from several hours or days in certain insects, for example, aphids, to 200 years, for example, in turtles. It is a species characteristic that is determined by evolution and is unrelated to the degree of organization or taxonomic position of the organism. Gerontology is the science in which changes that occur in middle and old age are studied. (SeeDETERMINATION, EMBRYONIC DEVELOPMENT, INDUCTORS, INDUCTION.)
REFERENCESIvanov, P. P. Obshchaia i sravnitel’naia embriologiia. Moscow-Leningrad, 1937.
Filatov, D. P. Sravnitel’no-morfologicheskoe napravlenie v mekhanike razvitiia, ego ob”ekt, tseli i puti. Moscow-Leningrad, 1939.
Darwin, C. “Proiskhozhdenie vidov putem estestvennogo otbora.” Soch, vol. 3. Moscow-Leningrad, 1939. (Translated from English.)
Timiriazev, K. A. “Istoricheskii metod v biologii.” Soch, vol. 6. Moscow, 1939.
Metchnikoff, E. O darvinizme (collection of articles). Moscow-Leningrad, 1943.
Zakhvatkin, A. A. Sravitel’naia embriologiia nizshikh bespozvonoch-nykh (istochniki i puti formirovaniia individual’nogo razvitiia mnogok-letochnykh), Moscow, 1949.
Takhtadzhian, A. L. Voprosy evoliutsionnoi morfologii rastenii. Leningrad, 1954.
Tokin, B. P. Obshchaia embriologiia, 2nd ed. Moscow, 1970.
Bodemer, C. Sovremennaia embriologiia. Moscow, 1971. (Translated from English.)
G. K. KHRUSHCHOV and N. G. KHRUSHCHOV
In plants. Early thoughts on plant ontogeny can be found in the works of the ancient scholars Theophrastus and Pliny the Elder. The scientific study of ontogeny was begun in the 18th century by C. Linnaeus (1751), J. W. Goethe (1790), and the Italian biologist P. Micheli (1729)—among others—and was continued in the 19th century by many scientists, including H. Dutrochet (1834), the Swiss algologist J. Vaucher (1803), and the French botanist G. Thuret (1853), all of whom studied the developmental cycles of algae and fungi. Ontogenetic patterns in higher plants were discovered by N. I. Zheleznov (1840), C. Naegeli (1842), M. Schleiden (1842–43), W. Hofmeister (1851), I. N. Gorozhankin (1880), V. I. Beliaev (1885), and S. G. Navashin (1898). In the second half of the 19th century, A. F. Batalin, M. S. Voronin, and J. Wiesner were among the many botanists who studied the relationship between ontogeny and habitat in different plant groups.
I. G. Gassner (1918) elucidated the role of low temperature in ear formation in winter grains, and W. W. Garner and H. A. Allard (1920) discovered photoperiodism. M. Kh. Chailakhian (1937) advanced the hormonal theory of flowering. The internal factors that affect ontogeny were explained by I. V. Michurin (1901–35), the German botanist W. Pfeffer (1904), the Austrian botanist H. Molisch (1929), and the Soviet botanist N. P. Krenke (1940). The morphological, physiological, biochemical, and genetic principles and evolution of ontogeny have been receiving close study since the mid-20th century.
Three major ontogenetic processes are distinguished in plants: growth, development, and aging. Growth is the formation of new structural elements that results in an increase in the mass and size of the organism. Development is the process during which the fertilized egg cell or vegetative rudiment acquires the shape of the adult organism as a result of cell division and differentiation and creates the specialized cells characteristic of the adult organism. Aging is the aggregate of irreversible structural, physiological, and biochemical changes that are due to a decrease in protein biosynthesis and in all physiological functions. It ultimately results in death of the organism.
Ontogeny is characterized by the close interaction of several aspects of a single process. Morphological aspects include morphogenesis—formation of the organism as a whole; organogenesis—shaping of individual organs; and histogenesis—formation of tissues. Physiological and biochemical aspects include all the physiological and biochemical processes that take place in the cells, tissues, and organs of a plant during development. The genetic aspect is the process by which hereditary information is realized. Ecological aspects are the growth and development of the organism in the environment, and the evolutionary aspect includes all changes in all aspects of ontogeny that originate in the long chain of generations at different phylogenetic stages. Thus, plant ontogeny is the product of long evolution; it is determined by the genotype and reflected in the successive series of physiological and biochemical processes that are responsible for the creation of morphological structures and that are required for the initiation of new, similar processes. Within the limits set by environmental conditions and the norms of reaction of the organism, the genotype is expressed in a series of pheno-types, each of which is characterized by ontogenetic periods during which new structures appear.
The principal feature of ontogeny in higher plants and in numerous species of algae is the alternation of asexual, called sporophyte, and sexual, called gametophyte, generations. The starting point for the formation of a sporophyte is a zygote, and for a gametophyte, a germinating spore. The development of the sporophyte and gametophyte is the aggregate of processes that end in the formation of the various organs. These processes vary among lower plants but constitute an ordered chain of events among higher plants. In ferns, for example, the sporophyte consists of an embryo, cormus, sporangium, and spore, while the gametophyte consists of a prothallium, archegonium or an-theridium, and egg cell or spermatozoid. In angiosperms, the gametophyte is greatly simplified.
In all stages of ontogeny, the organism is an integral system that is closely linked with the environment through both metabolism and the action of plant hormones. The transition from one stage of ontogeny to another is determined by the combined action of internal and external factors. The duration of ontogeny in plants varies from 20–30 minutes in bacteria to several thousand years in sequoias, junipers, and baobabs.
A knowledge of ontogeny is valuable for the sensible economic use of plants and for developing methods of increasing yields.
REFERENCESChailakhian, M. Kh. Osnovnye zakonomernosti onlogeneza vysshikh rastenii. Moscow, 1958.
Sabinin, D. A. Fiziologiia razvitiia rastenii. Moscow, 1963.
Skripchinskii, V. V. “Ocherk istorii fiziologii razvitiia rastenii.” In the collection Problemy fiziologii rastenii. Moscow, 1969.
Gupalo, P. I., and V. V. Skripchinskii. Fiziologiia individual’nogo razvitiia rastenii. Moscow, 1971.
Chailakhian, M. Kh., N. P. Aksenova, and V. I. Kefeli. O terminologii ontogeneza rastenii. Moscow, 1973.
V. V. SKRIPCHINSKII