Journey of Eric the Red. Eirik the Red, Scandinavian navigator: biography

Ontogenesis - the realization of genetic information occurring at all stages.

Ontogeny is a genetically controlled process. During ontogenesis, the genotype is realized and the phenotype is formed.

Ontogeny is the individual development of an organism, a set of successive morphological, physiological and biochemical transformations that an organism undergoes from the moment of its inception to the end of life. O. includes growth, i.e., an increase in body weight, its size, differentiation. The term "Oh." introduced by E. Haeckel(1866) when he formulated biogenetic law.

The first attempt at a historical substantiation of O. was made by I. f. Meckel. The problem of the relationship between O. and phylogenesis was posed by Ch. Darwin and developed by F. Muller,E. Haeckel, and others. All evolutionally new traits associated with changes in heredity arise in O., but only those that contribute to a better adaptation of the organism to the conditions of existence are preserved in the process. natural selection and are passed on to subsequent generations, that is, they are fixed in evolution. Knowledge of the patterns, causes, and factors of naturalization serves as the scientific basis for finding means of influencing the development of plants, animals, and humans, which is of great importance for the practice of crop and animal husbandry, as well as for medicine.

Phylogeny is the historical development of organisms. The term was introduced by evolutionist E. Haeckel in 1866. The main task in the study of F. is the reconstruction of the evolutionary transformations of animals, plants, microorganisms, establishing on this basis their origin and family ties between the taxa to which the studied organisms belong. For this purpose, E. Haeckel developed the method of "triple parallelism", which allows, by comparing the data of three sciences - morphology, embryology and paleontology - to restore the course of the historical development of the studied systematic group.

Law of germinal similarity

1. Embryos of animals of the same type in the early stages of development are similar.

2. They successively move in their development from more general features of the type to more and more particular ones. Lastly, signs develop that indicate that the embryo belongs to a particular genus, species, and, finally, individual traits.

3. Embryos of different representatives of the same type gradually separate from each other.

K. Baer, not being an evolutionist, could not connect the patterns of individual development discovered by him with the process of phylogenesis. Therefore, the generalizations he made had the value of no more than empirical rules.

The development of the evolutionary idea subsequently made it possible to explain the similarity of early embryos by their historical relationship, and the acquisition of more and more particular features by them with a gradual separation from each other - the actual isolation of the corresponding classes, orders, families, genera and species in the process of evolution.

Soon after the discovery of the law of germline similarity, Charles Darwin showed that this law testifies to the common origin and unity of the initial stages of evolution within a type.

biogenetic law Haeckel-Muller: every living being in its individual development ( ontogenesis) repeats to a certain extent the form passed by its ancestors or its species ( phylogenesis).

Ontogeny - repetition of phylogenesis

The repetition of structures characteristic of ancestors in the embryogenesis of descendants is called recapitulations. Recapitulate not only morphological features - the notochord, gill slit and gill arch anlages in all chordates, but also the features of the biochemical organization and physiology. Thus, in the evolution of vertebrates, there is a gradual loss of enzymes necessary for the breakdown of uric acid, a product of purine metabolism. In most invertebrates, the end product of the breakdown of uric acid is ammonia, in amphibians and fish it is urea, in many reptiles it is allantoin, and in some mammals uric acid is not broken down at all and is excreted in the urine. In the embryogenesis of mammals and humans, biochemical and physiological recapitulations were noted: the release of ammonia by early embryos, later urea, then allantoin, and, at the last stages of development, uric acid.

However, in the ontogeny of highly organized organisms, a strict repetition of the stages of historical development is not always observed, as follows from the biogenetic law. Thus, the human embryo never repeats the adult stages of fish, amphibians, reptiles and mammals, but is similar in a number of features only to their embryos. The early stages of development retain the greatest conservatism, due to which they recapitulate more completely than the later ones. This is due to the fact that one of the most important mechanisms of integration of the early stages of embryogenesis is embryonic induction, and the structures of the embryo that form in the first place, such as the notochord, neural tube, pharynx, intestine and somites, are the organizational centers of the embryo, from which the whole course of development depends.

The genetic basis of recapitulation lies in the unity of the mechanisms of genetic control of development, which is preserved on the basis of common genes for the regulation of ontogenesis, which are inherited by related groups of organisms from common ancestors.

Recapitulation(from Latin recapitulatio - repetition) - a concept used in biology to denote the repetition in individual development of features characteristic of an earlier stage of evolutionary development.

Ontogeny as the basis of phylogeny. Cenogenesis. Autonomization of ontogeny. Philembryogenesis. Teachings of A.N. Severtsov about phylembryogenesis. Mechanisms of their occurrence. Heterochrony and heterotopy of biological structures in the evolution of ontogeny.

Relying only on the basic biogenetic law, it is impossible to explain the process of evolution: the endless repetition of the past does not in itself give rise to a new one. Since life exists on Earth due to the change of generations of specific organisms, its evolution proceeds due to changes occurring in their ontogenies. These changes boil down to the fact that specific ontogenies deviate from the path laid by ancestral forms and acquire new features.

Such deviations include, for example, coenogenesis - adaptations that arise in embryos or larvae and adapt them to the characteristics of their habitat. In adult organisms, coenogenesis is not preserved. Examples of coenogenesis are horny formations in the mouth of tailless amphibian larvae, which make it easier for them to feed on plant foods. In the process of metamorphosis in the frog, they disappear and the digestive system is rebuilt to feed on insects and worms. The cenogenesis in amniotes includes the embryonic membranes, the yolk sac and allantois, and in placental mammals and humans, it also includes the placenta with the umbilical cord.

Cenogenesis, manifesting itself only in the early stages of ontogenesis, does not change the type of organization of the adult organism, but provides a higher probability of survival of the offspring. At the same time, they may be accompanied by a decrease in fertility and a lengthening of the embryonic or larval period, due to which the organism in the postembryonic or postlarval period of development is more mature and active. Having arisen and turned out to be useful, coenogenesis will be reproduced in subsequent generations. Thus, the amnion, which first appeared in the ancestors of reptiles in the Carboniferous period of the Paleozoic era, is reproduced in all vertebrates that develop on land, both in egg-laying reptiles and birds, and in placental mammals.

Another type of phylogenetically significant transformations of phylogeny is phylembryogenesis. They represent deviations from the ontogeny characteristic of ancestors, manifested in embryogenesis, but having an adaptive significance in adult forms. Thus, the anlage of the hairline appears in mammals at very early stages of embryonic development, but the hairline itself is important only in adult organisms.

Such changes in ontogeny, being useful, are fixed by natural selection and reproduced in subsequent generations. These changes are based on the same mechanisms that cause congenital malformations: a violation of cell proliferation, their movement, adhesion, death or differentiation (see § 8.2 and 9.3). However, they, like cenogenesis, are distinguished from malformations by adaptive value, i.e. usefulness and fixation by natural selection in phylogenesis.

Depending on the stages of embryogenesis and morphogenesis of specific structures, developmental changes that have the significance of phylembryogenesis occur, three types of them are distinguished.

1.Anabolia, or extensions, appear after the organ has almost completed its development, and are expressed in the addition of additional stages that change the final result.

Anabolisms include such phenomena as the acquisition of a specific body shape by a flounder only after a fry hatches from an egg, indistinguishable from other fish, as well as the appearance of spine bends, fusion of sutures in the brain skull, the final redistribution of blood vessels in the body of mammals and humans.

2.Deviations - deviations arising in the process of organ morphogenesis. An example is the development of the heart in the ontogeny of mammals, in which it recapitulates the tube stage, two-chamber and three-chamber structure, but the stage of formation of an incomplete septum, characteristic of reptiles, is supplanted by the development of a septum, built and located differently and characteristic only of mammals (see § 14.4) .In the development of the lungs in mammals, recapitulation of the early stages of ancestors is also found, later morphogenesis proceeds in a new way (see Section 14.3.4).

Rice. 13.9. Transformations of onto- and phylogenesis in connection with emerging phylembryogenesis

The letters indicate the stages of ontogenesis, the numbers indicate phylembryogenetic transformations.

3.Archallaxis - changes that are found at the level of rudiments and are expressed in a violation of their division, early differentiation, or in the appearance of fundamentally new anlages. A classic example of archallaxis is

the development of hair in mammals, the anlage of which occurs at a very early stage of development and differs from the very beginning from the anlage of other vertebrate skin appendages (see § 14.1).

According to the type of archallaxis, a notochord arises in primitive non-cranial animals, a cartilaginous spine in cartilaginous fish (see section 14.2.1.1), nephrons of the secondary kidney develop in reptiles (see section 14.5.1).

It is clear that during evolution due to anabolism, the main biogenetic law is fully realized in the ontogenies of descendants, i.e. recapitulations of all ancestral stages of development occur. In deviations, the early ancestral stages recapitulate, while the later ones are replaced by development in a new direction. Archallaxis completely prevent recapitulation in the development of these structures, changing their very beginnings.

If we compare the diagram of phylembryogenesis with the table of K. Baer (Fig. 13.9), illustrating the law of germline similarity, it will become clear that Baer was already very close to the discovery of phylembryogenesis, but the absence of an evolutionary idea in his reasoning did not allow him to be more than 100 years ahead of scientific thought .

In the evolution of ontogeny, anabolisms are most often encountered as phylembryogenesis, which only to a small extent change the integral process of development. Deviations as violations of the morphogenetic process in embryogenesis are often swept aside by natural selection and therefore occur much less frequently. Archallaxis appear most rarely in evolution due to the fact that they change the entire course of embryogenesis, and if such changes affect the rudiments of vital organs or organs that are important as embryonic organizational centers (see Section 8.2.6), then they often turn out to be incompatible with life.

In the same phylogenetic group, evolution in different organ systems can occur due to different phylembryogenesis.

Thus, in the ontogeny of mammals, all stages of the development of the axial skeleton in the subtype of vertebrates (anabolism) are traced, in the development of the heart, only early stages recapitulate (deviation), and in the development of skin appendages there are no recapitulations at all (archallaxis). Knowledge of the types of phylembryogenesis in the evolution of chordate organ systems is necessary for a doctor to predict the possibility of atavistic birth defects in fetuses and newborns (see Section 13.3.4). Indeed, if atavistic malformations are possible in an organ system that evolves through anabolism and deviations due to the recapitulation of ancestral states, then in the case of archallaxis this is completely excluded.

In addition to cenogenesis and phylembryogenesis, in the evolution of ontogenesis, deviations in the time of laying organs can also be detected - heterochrony - and places of their development - heterotopias. Both the first and the second lead to a change in the relationship of developing structures and are subject to strict control of natural selection. Only those heterochronies and heterotopias are preserved that are useful. Examples of such adaptive heterochrony are shifts in time of the anlage of the most vital organs in groups evolving according to the type of arogenesis. Thus, in mammals, and especially in humans, the differentiation of the forebrain significantly outstrips the development of its other departments.

Heterotopies lead to the formation of new spatial and functional relationships between organs, ensuring their joint evolution in the future. So, the heart, located in fish under the pharynx, provides an effective supply of blood to the gill arteries for gas exchange. Moving to the retrosternal region in terrestrial vertebrates, it develops and functions already in a single complex with new respiratory organs - the lungs, performing here, first of all, the function of delivering blood to the respiratory system for gas exchange.

Heterochronies and heterotopies, depending on the stages of embryogenesis and morphogenesis of organs, can be regarded as different types of phylembryogenesis. Thus, the movement of the rudiments of the brain, leading to its bending, characteristic of amniotes, and manifesting itself at the initial stages of its differentiation, is archallaxis, and heterotopia of the testis in humans from the abdominal cavity through the inguinal canal to the scrotum, observed at the end of embryogenesis after its final formation, - typical anabolic.

Sometimes processes of heterotopy, identical in results, can be phylembryogenesis of different types. For example, in various classes of vertebrates, movement of the limb belts is very common. In many groups of fish leading a benthic lifestyle, the ventral fins (hind limbs) are located anterior to the pectorals, while in mammals and humans, the shoulder girdle and forelimbs in the definitive state are much caudal to the place of their initial laying. In this regard, the innervation of the shoulder girdle in them is carried out by nerves associated not with the thoracic, but with the cervical segments of the spinal cord. In the fish mentioned above, the ventral fins are innervated not by the nerves of the posterior trunk, but by the anterior segments located anterior to the centers of innervation of the pectoral fins. This indicates the heterotopy of the fin anlage already at the stage of the earliest rudiments, while the movement of the anterior girdle of the limbs in humans occurs at later stages, when their innervation is already fully realized. Obviously, in the first case, heterotopy is archallaxis, while in the second, it is anabolic.

Cenogenesis, phylembryogenesis, as well as heterotopy and heterochrony, having proved useful, are fixed in the offspring and reproduced in subsequent generations until new adaptive changes in ontogenesis displace them, replacing them. Due to this, ontogeny not only briefly repeats the evolutionary path traversed by the ancestors, but also paves the way for new directions of phylogenesis in the future.

Cenogenesis

(from Greek kainós - new and ... genesis (See ... genesis)

adaptation of an organism that occurs at the stage of the embryo (fetus) or larva and is not preserved in an adult. Examples C. - the placenta of mammals, providing the fetus with respiration, nutrition and excretion; external gills of amphibian larvae; an egg tooth in birds, which serves to chicks to break through the egg shell; attachment organs in the larva of ascidians, a swimming tail in the larva of trematodes - cercaria, etc. The term "C." introduced in 1866 by E. Haeckel to designate those characters that, by violating the manifestations of palingenesis (See. Palingenesis), i.e. repetitions of distant stages of phylogenesis in the process of embryonic development of an individual do not allow us to trace the sequence of stages of phylogenesis of their ancestors during the ontogenesis of modern forms, i.e. violate biogenetic law. At the end of the 19th century C. began to be called any change in the course of ontogenesis characteristic of ancestors (German scientists E. Mehnert, F. Keibel, and others). The modern understanding of the term "C." was formed as a result of the work of A. N. Severtsov, who retained for this concept only the meaning of provisional adaptations, or embryo-adaptation. see also Philembryogenesis.

Cenogenesis(Greek kainos new + genesis birth, formation) - the appearance in the embryo or larva of adaptations to the conditions of existence that are not characteristic of adult stages, for example. the formation of membranes in the embryos of higher animals.

Philembryogenesis

(from the Greek phýlon - tribe, genus, species and Embryogenesis

FILEMBRIOGENESIS (from Greek phylon - genus, tribe, embryon - embryo and genesis - origin), evolutionary change ontogeny organs, tissues and cells, associated with both progressive development and reduction. The doctrine of phylembryogenesis was developed by a Russian evolutionary biologist A.N. Severtsov. The modes (methods) of phylembryogenesis differ in the time of occurrence in the process of development of these structures.

If the development of a certain organ in the descendants continues after the stage at which it ended in the ancestors, anabolism occurs (from the Greek anabole - rise) - an extension of the final stage of development. An example is the formation of a four-chambered heart in mammals. Amphibians have a three-chambered heart: two atria and one ventricle. In reptiles, a septum develops in the ventricle (the first anabolism), but this septum is incomplete in most of them - it only reduces the mixing of arterial and venous blood. In crocodiles and mammals, the development of the septum continues until the complete separation of the right and left ventricles (second anabolism). In children, sometimes as an atavism, the interventricular septum is underdeveloped, which leads to a serious illness requiring surgical intervention.

Prolongation of the development of an organ does not require profound changes in the previous stages of its ontogenesis; therefore, anabolism is the most common method of phylembryogenesis. The stages of organ development preceding anabolism remain comparable to the stages phylogenesis ancestors (i.e. are recapitulations) and can serve for its reconstruction (see Fig. biogenetic law). If the development of an organ at intermediate stages deviates from the path along which its ontogeny went in its ancestors, a deviation occurs (from late Latin deviatio - deviation). For example, in fish and reptiles, scales appear as thickenings of the epidermis and the underlying connective tissue layer of the skin - the corium. Gradually thickening, this bookmark bends outward. Then, in fish, the corium ossifies, the forming bony scale pierces the epidermis and extends to the surface of the body. In reptiles, on the contrary, the bone does not form, but the epidermis becomes keratinized, forming the horny scales of lizards and snakes. In crocodilians, the corium can ossify, forming the bony base of the horny scales. Deviations lead to a deeper restructuring of ontogeny than anabolism, so they are less common.

Least of all, changes in the primary rudiments of organs occur - archallaxis (from the Greek arche - beginning and allaxis - change). With deviation, recapitulation can be traced from the laying of the organ to the moment of deviation of development. With archallaxis, there is no recapitulation. An example is the development of the vertebral bodies in amphibians. In fossil amphibians - stegocephals and in modern tailless amphibians, the vertebral bodies form around a chord of several, usually three on each side of the body, separate anlages, which then merge to form the vertebral body. In tailed amphibians, these bookmarks do not occur. The ossification grows from above and below, covering the chord, so that a bone tube is immediately formed, which, thickening, becomes the body of the vertebra. This archallaxis is the cause of the still debated question of the origin of the tailed amphibians. Some scientists believe that they descended directly from lobe-finned fish, independently of other terrestrial vertebrates. Others - that the tailed amphibians very early diverged from the rest of the amphibians. Still others, neglecting the development of the vertebrae, prove the close relationship of the caudate and anuran amphibians.

Organ reduction, which have lost their adaptive significance, also occurs through phylembryogenesis, mainly through negative anabolism - the loss of the final stages of development. In this case, the organ either underdeveloped and becomes rudiment, or undergoes a reverse development and completely disappears. An example of a rudiment is the human appendix - an underdeveloped caecum, an example of complete disappearance - the tail of frog tadpoles. Throughout life, the tail grows in water, new vertebrae and muscle segments are added at its end. During metamorphosis, when the tadpole turns into a frog, the tail dissolves, and the process goes in the reverse order - from the end to the base. Phylembryogenesis is the main way of adaptive changes in the structure of organisms during phylogenesis.

Principles (methods) of phylogenetic transformations of organs and functions. Correspondence of structure and function in living systems. Polyfunctionality. Quantitative and qualitative changes in the functions of biological structures.

GENERAL REGULARITIES

THE EVOLUTION OF ORGANS

An organism, or an individual, is a separate living being, in the process of ontogenesis, showing all the properties of a living thing. The constant interaction of an individual with the environment in the form of organized flows of energy and matter maintains its integrity and development. Structurally, the body is an integrated hierarchical system built from cells, tissues, organs and systems that ensure its vital activity. Let us dwell on the organs and life support systems in more detail.

Authority called a historically established specialized system of tissues, characterized by delimitation, constancy of shape, localization, internal structure of the blood circulation and innervation pathways, development in ontogenesis and specific functions. The structure of organs is often very complex. Most of them are polyfunctional, i.e. performs several functions at the same time. At the same time, various organs may be involved in the implementation of any complex function.

A group of organs of similar origin that combine to perform a complex function is called system(circulatory, excretory, etc.).

If the same function is performed by a group of organs of different origin, it is called apparatus. An example is the respiratory apparatus, consisting of both the respiratory organs themselves and the elements of the skeleton and muscular system that provide respiratory movements.

In the process of ontogenesis, development occurs, and often the replacement of some organs by others. The organs of a mature organism are called definitive; organs that develop and function only in embryonic or larval development, - provisional. Examples of provisional organs are the gills of amphibian larvae, the primary kidney, and the embryonic membranes of higher vertebrates (amniotes).

In historical development, transformations of organs may be progressive or regressive. In the first case, the organs increase in size and become more complex in structure, in the second, they decrease in size, and their structure is simplified.

If two organisms at different levels of organization have organs that are built according to a single plan, located in the same place and develop in a similar way from the same embryonic rudiments, then this indicates the relationship of these organisms. Such bodies are called homologous. Homologous organs often perform the same function (for example, the heart of fish, amphibian, reptile and mammal), but in the process of evolution, functions may change (for example, the forelimbs of fish and amphibians, reptiles and birds).

When unrelated organisms live in the same environment, they may develop similar adaptations, which manifest themselves in the appearance similar organs. Similar organs perform the same functions, but their structure, location and development are sharply different. Examples of such organs are the wings of insects and birds, the limbs and jaw apparatus of arthropods and vertebrates.

The structure of organs strictly corresponds to the functions they perform. At the same time, in the historical transformations of organs, a change in functions is invariably accompanied by a change in the morphological characteristics of the organ.

Researchers in the early 19th century For the first time, attention began to be paid to the similarity of the stages of development of the embryos of higher animals with the stages of complication of organization, leading from low-organized forms to progressive ones. Comparing the stages of development of embryos of different species and classes of chordates, K. Baer made the following conclusions.
1. Embryos of animals of the same type in the early stages of development are similar.
2. They successively move in their development from more general features of the type to more and more particular ones. Lastly, signs develop that indicate that the embryo belongs to a particular genus, species, and, finally, individual traits.
3. Embryos of different representatives of the same type gradually separate from each other (Fig. 13.8).
K. Baer, not being an evolutionist, could not connect the patterns of individual development discovered by him with the process of phylogenesis. Therefore, the generalizations he made had the value of no more than empirical rules.
The development of the evolutionary idea subsequently made it possible to explain the similarity of early embryos by their historical relationship, and the acquisition of more and more particular features by them with a gradual isolation from each other - the actual isolation of the corresponding classes, orders, families, genera and species in the process of evolution.
Soon after the discovery of the law of germline similarity, Charles Darwin showed that this law testifies to the common origin and unity of the initial stages of evolution within a type.

Rice. 13.8. Similarity of embryos of different classes of vertebrates at different stages (I-Ill) of ontogeny
13.2.2. Ontogeny - repetition of phylogenesis

Comparing the ontogenesis of crustaceans with the morphology of their extinct ancestors, F. Müller concluded that living crustaceans in their development repeat the path traveled by their ancestors. The transformation of ontogeny into evolution, according to F. Muller, is carried out due to its elongation by adding additional stages or extensions to it. On the basis of these observations, as well as the study of the development of chordates, E. Haeckel (1866) formulated the basic biogenetic law, according to which ontogenesis is a brief and rapid repetition of phylogenesis.
The repetition of structures characteristic of ancestors in the embryogenesis of descendants is called recapitulations. Recapitulate not only morphological characters - notochord, gill slit and gill arch anlages in all chordates, but also features of biochemical organization and physiology. Thus, in the evolution of vertebrates, there is a gradual loss of enzymes necessary for the breakdown of uric acid, a product of purine metabolism. In most invertebrates, the end product of the breakdown of uric acid is ammonia, in amphibians and fish it is urea, in many reptiles it is allantoin, and in some mammals uric acid is not broken down at all and is excreted in the urine. In the embryogenesis of mammals and humans, biochemical and physiological recapitulations were noted: the release of ammonia by early embryos, later urea, then allantoin, and, at the last stages of development, uric acid.
However, in the ontogeny of highly organized organisms, a strict repetition of the stages of historical development is not always observed, as follows from the biogenetic law. Thus, the human embryo never repeats the adult stages of fish, amphibians, reptiles and mammals, but is similar in a number of features only to their embryos. The early stages of development retain the greatest conservatism, due to which they recapitulate more completely than the later ones. This is due to the fact that one of the most important mechanisms of integration of the early stages of embryogenesis is embryonic induction, and the structures of the embryo that form in the first place, such as the notochord, neural tube, pharynx, intestine and somites, are the organizational centers of the embryo, from which the whole course of development depends.
The genetic basis of recapitulation lies in the unity of the mechanisms of genetic control of development, which is preserved on the basis of common genes for the regulation of ontogenesis, which are inherited by related groups of organisms from common ancestors.

The law of germline similarity K. Baer

1. Embryos of animals of the same type in the early stages of development are similar.

3. Embryos of different representatives of the same type gradually separate from each other (Fig. 1).

Fig.1. Similarity of embryos of different classes of vertebrates at different stages of development

The development of the evolutionary idea subsequently made it possible to explain the similarity of early embryos by their historical relationship, and the acquisition of more and more particular features by them with a gradual isolation from each other - the actual isolation of the corresponding classes, orders, families, genera and species in the process of evolution.

The evolutionary doctrine developed by Charles Darwin brightly highlighted the fundamental significance of the problem of ontogenetic development. The germline similarity is now explained by the actual relationship of organisms, and their gradual divergence (embryonic divergence) is an obvious reflection of the historical divergence of these forms (phylogenetic divergence). In the germ of descendants, Darwin wrote, we see a "vague portrait" of ancestors. Therefore, the history of a given species can be traced by individual development.

E. Haeckel's biogenetic law

F. Muller in his work "For Darwin" (1864) formulated the idea that changes in ontogenetic development underlying the process of evolution can be expressed in changes in the early or late stages of organ development. In the first case, only the general similarity of young embryos is preserved. In the second case, an extension and complication of ontogeny is observed, associated with the addition of stages and repetition (recapitulation) in the individual development of traits of more distant adult ancestors.
Muller's work served as the basis for the formulation by E. Haeckel (1866) of the basic biogenetic law, according to which ontogeny is a brief and rapid repetition of phylogeny. That is, an organic individual repeats during the rapid and short course of its individual development the most important of those changes in form that its ancestors went through during the slow and long course of their paleontological development according to the laws of heredity and variability. The signs of adult ancestors, which are repeated in the embryogenesis of descendants, he called palingenesis. Adaptations to the embryonic or larval stages are called coenogenesis.

However, Haeckel's ideas were very different from Muller's views on the question of the relationship between ontogenesis and phylogenesis in the process of evolution. Müller believed that evolutionarily new forms arise by changing the course of individual development characteristic of their ancestors, i.e. changes in ontogeny are primary in relation to phylogenetic changes. According to Haeckel, on the contrary, phylogenetic changes precede changes in individual development. Evolutionary new signs arise not during ontogeny, but in an adult organism. An adult organism evolves, and in the process of this evolution, the signs are shifted to earlier stages of ontogeny. Thus, the problem of the relationship between ontogenesis and phylogenesis arose, which has not been resolved to this day.

Haeckel, in contrast to Müller, believed that phylogenesis is influenced only by the elongation of ontogeny through the addition of stages, while all other stages remain unchanged. Consequently, Haeckel accepted only the second path of historical changes in ontogenesis (according to Müller) and left aside the change in the stages of ontogenesis themselves as the basis of phylogenetic transformations. It was on this form of interdependence of ontogeny and phylogenesis that Darwin and Müller emphasized. The interpretation of the biogenetic law in the understanding of Muller was later developed by A.N. Severtsov (1910-1939) in the theory of phylembryogenesis. Severtsov shared Müller's views on the primacy of ontogenetic changes in relation to changes in adult organisms and considered ontogeny not only as the result of phylogenesis, but also as its basis. Ontogeny is not only lengthened by the addition of stages: it is entirely restructured in the process of evolution; it has its own history, naturally connected with the history of the adult organism and partly determining it.

Phylembryogenesis is the embryonic changes associated with the phylogenetic development of an adult organism. In the process of evolution, all stages of development are rebuilt. New changes often occur at the last stages of shaping. Ontogeny is complicated by the addition, or extension, stages (anabolism). Only in this case are there all the prerequisites for the repetition in ontogenesis of the historical stages in the development of these parts in distant ancestors (recapitulation). Ontogeny can, however, change at any other stages of development, while deviating all later stages from the previous path (deviation). Finally, it is also possible to change the very rudiments of organs or parts (archallaxis). Then the whole ontogeny turns out to be changed, and in the individual development of the descendants there are no indications of the sequence of passing through the historical stages of development of their ancestors.

The biological essence of E. Haeckel's biogenetic law

Haeckel's biogenetic law and Severtsov's theory of phylembryogenesis play an important role in the development of morphology and evolutionary theory itself. The study of the individual development of animals has provided ample evidence of their historical development. The biogenetic law is an important component of the triple parallelism method developed by E. Haeckel, with the help of which phylogenesis is reconstructed. This method is based on a comparison of morphological, embryological, and paleontological data. Morphologists in the reconstruction of phylogeny still use Haeckel's principle, according to which the ontogeny of descendants briefly repeats, recapitulates the stages of the phylogeny of ancestors. Relying only on the basic biogenetic law, it is impossible to explain the process of evolution: the endless repetition of the past does not in itself give rise to a new one. Since life exists on Earth due to the change of generations of specific organisms, its evolution proceeds due to changes occurring in their ontogenies. These changes boil down to the fact that specific ontogenies deviate from the path laid by ancestral forms and acquire new features.

Such deviations include, for example, cenogenesis - adaptations that arise in embryos or larvae and adapt them to the characteristics of the habitat. In adult organisms, coenogenesis is not preserved. Examples of coenogenesis are horny formations in the mouth of tailless amphibian larvae, which make it easier for them to feed on plant foods. In the process of metamorphosis in the frog, they disappear and the digestive system is rebuilt to feed on insects and worms. To cenogenesis in placental mammals and humans - the placenta with the umbilical cord.

Another type of phylogenetically significant transformations of phylogenesis is phylembryogenesis. They represent deviations from the ontogeny characteristic of ancestors, manifested in embryogenesis, but having an adaptive significance in adult forms. Thus, the anlage of the hairline appears in mammals at very early stages of embryonic development, but the hairline itself is important only in adult organisms.
Such changes in ontogeny, being useful, are fixed by natural selection and reproduced in subsequent generations. These changes are based on the same mechanisms that cause congenital malformations: violation of cell proliferation, their movement, adhesion, death or differentiation. However, just like cenogenesis, they are distinguished from vices by their adaptive value, i.e. usefulness and fixation by natural selection in phylogenesis.

Depending on the stages of embryogenesis and morphogenesis of specific structures, developmental changes that have the significance of phylembryogenesis occur, three types of them are distinguished.

1. Anabolia, or extensions, occur after the organ has almost completed its development, and are expressed in the addition of additional stages that change the final result. Anabolisms include such phenomena as the acquisition of a specific body shape by a flounder only after a fry hatches from an egg, indistinguishable from other fish, as well as the appearance of spine bends, fusion of sutures in the brain skull, the final redistribution of blood vessels in the body of mammals and humans.

2. Deviations - deviations arising in the process of organ morphogenesis. An example may be the development of the heart in the ontogeny of mammals, in which it recapitulates the tube stage, a two-chamber and three-chamber structure, but the stage of formation of an incomplete septum, characteristic of reptiles, is supplanted by the development of a septum, built and located differently and characteristic only for mammals. In the development of the lungs in mammals, recapitulation of the early stages of the ancestors is also found, later morphogenesis proceeds in a new way.

3. Archallaxis - changes that are found at the level of rudiments and are expressed in a violation of their dissection, early differentiation, or in the appearance of fundamentally new anlages. A classic example of archallaxis is the development of hair in mammals, the anlage of which occurs at very early stages of development and differs from the anlage of other vertebrate skin appendages from the very beginning. According to the type of archallaxis, a notochord arises in primitive non-cranial animals, a cartilaginous spine in cartilaginous fish, and nephrons of the secondary kidney develop in reptiles.

In the evolution of ontogeny, anabolisms are most often encountered as phylembryogenesis, which only to a small extent change the integral process of development. Deviations as violations of the morphogenetic process in embryogenesis are often swept aside by natural selection and therefore occur much less frequently. Archallaxis appear most rarely in evolution due to the fact that they change the entire course of embryogenesis, and if such changes affect the rudiments of vital organs or organs that are important as embryonic organizational centers, then they often turn out to be incompatible with life.

In addition to cenogenesis and phylembryogenesis, in the evolution of ontogeny, deviations in the time of laying organs - heterochrony - and the place of their development - heterotopia can also be detected. Both the first and the second lead to a change in the relationship of developing structures and are subject to strict control of natural selection. Only those heterochronies and heterotopias are preserved that are useful. Examples of such adaptive heterochrony are shifts in time of the anlage of the most vital organs in groups evolving according to the type of arogenesis. Thus, in mammals, and especially in humans, the differentiation of the forebrain significantly outstrips the development of its other departments.

Heterochronies and heterotopies, depending on the stages of embryogenesis and morphogenesis of organs, can be regarded as different types of phylembryogenesis. Testicular heterotopia in humans from the abdominal cavity through the inguinal canal to the scrotum, observed at the end of embryogenesis after its final formation, is a typical anabolism.

The law of germinal similarity.

Carl von Baer formulated his ideas about the similarities between the embryos of different classes of vertebrates in the form of four positions:

"In every large group, the general is formed before the special."
"The less general is formed from the universal, and so on, until, finally, the most special appears."
"Each embryo of a certain animal form, instead of passing through other certain forms on the contrary, departs from them.
"A higher form embryo never resembles another animal form, but only its embryos."

The last pattern, referring to Baer, was used by Charles Darwin as one of the proofs of evolution and gave it the name "law of germinal resemblance".

In 1828, Baer formulated a pattern, which is called Baer's Law: "The earlier stages of individual development are compared, the more similarities can be found." This great embryologist noticed that the embryos of mammals, birds, lizards, snakes and other terrestrial vertebrates in the early stages of development are very similar to each other both in general and in the way of development of their parts. The legs of a lizard, the wings and legs of birds, the limbs of mammals, as well as the arms and legs of man, develop, as Baer noted, in a similar way and from the same rudiments. Only with further development in the embryos of different classes of vertebrates do differences appear - signs of classes, orders, genera, species, and, finally, signs of a given individual.

biogenetic law.

For the first time, the relationship between ontogenesis and phylogenesis in a number of provisions was revealed by K. Baer, to which Charles Darwin gave the generalized name of the "law of germinal similarity." In the germ of descendants, Darwin wrote, we see a "vague portrait" of ancestors. In other words, already at the early stages of embryogenesis of different species within the limits of the type, a great similarity is revealed. Therefore, the history of a given species can be traced by individual development.

The most pronounced germline similarity in the early stages. In the later stages, embryonic divergence is observed, reflecting the divergence in the evolution of these species.

In 1864, F. Müller formulated the idea that phylogenetic transformations are associated with ontogenetic changes and that this connection manifests itself in two different ways. In the first case the individual development of descendants proceeds similarly to the development of ancestors only until a new trait appears in ontogeny. The change in the processes of morphogenesis of descendants determines that their embryonic development repeats the history of their ancestors only in general terms. In the second case descendants repeat the entire development of their ancestors, but by the end of embryogenesis new stages are added, as a result of which the embryogenesis of descendants is lengthened and more complicated. The repetition of signs of adult ancestors in the embryogenesis of descendants F. Muller called recapitulation.

Müller's works served as the basis for E. Haeckel's formulation biogenetic law, according to which ontogeny there is a brief and quick recapitulation of the phylogeny. The signs of adult ancestors that are repeated in the embryogenesis of descendants, he called palingenesis. These include in amniotes the separation of primary germ layers, the formation of a primary cartilaginous skull, gill arches, and a single-chamber heart. Adaptations to the embryonic or larval stages are called coenogenesis. Among them is the formation of a nutritious yolk in the egg and in the egg membranes, amnion and allantois. According to E. Haeckel, coenogenesis (embryonic adaptations) distort, or, as he put it, "falsify" the complete repetition of the history of ancestors in embryogenesis and represent a phenomenon secondary to recapitulation.

In the interpretation of the biogenetic law by E. Haeckel, phylogenesis is influenced only by the elongation of ontogeny by adding stages, while all other stages remain unchanged. Consequently, Haeckel accepted only the second path of historical changes in ontogenesis (according to Müller) and left aside the change in the stages of ontogenesis themselves as the basis of phylogenetic transformations. It was on this form of interdependence of ontogeny and phylogenesis that Darwin and Müller emphasized. The interpretation of the biogenetic law in the understanding of C. Darwin and F. Muller was later developed by A. N. Severtsov in the theory phylembryogenesis.

Thus, ontogeny is not only the result, but also the basis of phylogeny. Ontogeny is transformed in different ways: by restructuring existing stages and by adding new stages. Phylogeny cannot be regarded as the history of only adult organisms. This process is a historical chain of transforming ontogenies.