Epigenesis

EPIGENESIS (from the Greek epigenesis—after-birth, after-formation), the doctrine of embryonic development as a process realized through the sequential appearance of new formations and diversity, in contrast to the doctrine of preformation (see),—the pre-existence in the embryo of an initial diversity. In view of the fact that at different times and by its various representatives, diametrically opposite content was sometimes invested in the doctrine of epigenesis, it is necessary to strictly distinguish between the historical and modern theoretical understanding of this doctrine. Historical roots of the doctrine of epigenesis. From the very first steps of the doctrine of embryonic development, the epigenetic point of view gained predominance. Speaking in its favor as an obvious fact were the already accidental and unsystematic observations of animal and human embryos, which were known even to the ancient Greeks. Aristotle himself performed a classic experiment of sequentially breaking open and studying chicken eggs being incubated day by day. Aristotle believed that the heart is the first to form in the embryo, the internal parts earlier than the external, the anterior earlier than the posterior. To him also belongs the first theoretical justification of the doctrine of epigenesis. At the same time, Aristotle subjected to witty criticism another view common in his time, analogous to the theory later named (by Darwin) pangenesis (see). Hippocrates, the most famous proponent of the latter doctrine, believed that in the seed, the presence of which he admitted in both the male and female sex, particles from the whole body are collected; upon the mixing of male and female seed, which occurs during the act of fertilization, these particles are also mixed. By the mixing and struggle (“force”) of these elements, it seemed possible to explain all phenomena of development and heredity. It is in Hippocrates that we first encounter a theoretical justification for the inheritance of acquired characteristics. However, it would be erroneous to assume that the doctrines of pangenesis and epigenesis were mutually exclusive and obligatory antagonists. The doctrine of pangenesis, not only in antiquity but also in the 17th and 18th centuries, was usually combined with the doctrine of subsequent epigenesis and served as a kind of first primitive attempt at explaining the processes of development. Only at the end of the 19th century did pangenesis and, in general, all the numerous theories of vital and hereditary representative units turn out to gravitate toward preformationist interpretations. In contrast to Hippocrates, Aristotle admitted the materiality and physical participation in the development of the embryo only of the female seed (originating from menstrual blood); the male seed, which is in essence the only genuine seed, although it is the primary and necessary source, the cause of development and the animation of the embryo, does not materially participate in the formation of the latter. This spiritual, vitalistic principle of development, from the time of Aristotle and partly thanks to his enormous authority, appeared as an almost mandatory element of all subsequent theories of epigenesis up until the 19th century, and in the 20th century, it was revived in the form of Driesch's theory of epigenetic development directed by entelechy. The teachings of the Arab commentators of Aristotle, Christian scholasticism, and the mysticism of the Middle Ages prepared the elements of the future theory of preformation. However, the first embryologists of the new era (Coiter, 1572; Fabricius, 1600, 1621) shared the epigenetic point of view and attempted, on the same classic object—the chicken egg—to trace the sequence of the appearance of individual organs. While Aldrovandi (1600) admitted the direct material participation of the male seed in the formation of the embryo (the chalazae in the chicken egg), Fabricius, although he described the first stages of development of the chicken egg, stood entirely on the Aristotelian point of view regarding the question of the participation of the male seed. Furthermore, Fabricius erroneously believed that the chicken embryo was localized in the chalazae of the white. Only Parisanus (1621), who confirmed Coiter's observations, drew attention to the white spot (blastoderm) on the yolk as the place where the development of the embryo begins; he believed that this was the rooster's seed, thus representing the germ of the embryo. Harvey (1651) is usually credited with the justification of the doctrine of epigenesis. He put forward the principle—"everything from the egg" (ex ovo omnia). However, it should be borne in mind that by an egg, Harvey understood the eggs of oviparous animals, whereas in mammals he analogized the embryo discovered by him (in a roe deer) a month after conception to an egg; finally, in insects, Harvey took the larva and pupa for an egg. As for the role of the male seed, Harvey also stands on the traditional, Aristotelian point of view. The male seed acts on the egg not by material contact, but at a distance, evaporating, turning into a seminal essence. Borrowing entirely the point of view of Aristotle, Harvey believes that in the seed is the primary immaterial cause and the moving principle of development. Harvey proposed the term epigenesis for development by the “addition of parts, separating one after another.” However, Harvey accepts development by epigenesis only for higher animals, while lower ones, e.g., insects, are formed by “metamorphosis,” in which all parts appear simultaneously and at once. This strange dualism of Harvey is explained by his aforementioned erroneous ideas about the analogs of the egg in lower animals (larvae and pupae of insects). The brilliant galaxy of observers after Harvey (Swammerdam, Malpighi, de Graaf, Leeuwenhoek) far surpassed the latter in the detail and thoroughness of their descriptions. It is not surprising, therefore, that when the theory of preformation (see) gained justification on the basis of their observations, made already with the use of a microscope, it was perceived as a genuine scientific discovery. The theory of epigenesis was almost entirely abandoned as completely incapable of scientifically explaining the process of development. It is often believed that the triumph of preformation at the end of the 17th and the beginning of the 18th centuries is explained by the spread of the scientific mechanistic worldview that flourished by this time (Galileo, Descartes, Leibniz). However, such an explanation is simplified. On the contrary, proponents of preformation (Bonnet, Haller) often defended the point of view of the integrity of the organism and expressed bewilderment at the possibility of explaining development by the mechanical sequential summation of individual organs. In an organism, all organs are so interconnected that they can appear only simultaneously—can, for example, blood vessels exist without a heart, etc.? But preformation encountered other insurmountable obstacles—under its assumption, it was impossible to explain either the phenomena of the variability of embryos under the influence of external factors (e.g., the appearance of monsters), or the phenomena of hybridization (the mule, combining the characteristics of both the horse and the donkey), or, finally, the phenomena of regeneration. It goes without saying that preformation extremely hindered the development of descriptive embryological research. Indeed, despite the fact that the development of, for example, a chicken in an egg had been studied since antiquity and a number of very good descriptions of it had been given in the new era (Fabricius, 1621; Highmore, 1651; Malpighi, 1673; Haller, 1758), only C. F. Wolff (1733–1794) laid the foundation for a correct scientific understanding of the processes observed therein. In the doctoral dissertation of the budding young scientist (“Theoria generationis,” 1759), a philosophical and scientific justification for epigenesis was given. Wolff asserts that in an unincubated egg and, in general, in any germ that has not begun development, nothing can be found except a liquid, transparent, and structureless mass. Subsequently, the flows of “juices” and the movement of “droplets” and “globules” cause the sequential formation of channels, vessels, and other parts of the developing embryo. Wolff apparently managed to partially observe cellular structures, especially in plants, the study of the structure and development of which he draws upon for the understanding of the processes of animal development. In his further, more detailed work devoted to the development of the intestine of the chicken (1768–1769), Wolff lays the foundation for the theory of “germ layers,” asserting that the main organs develop from leaf-like plates, which, through various mechanical processes, form tubes, cavities, etc. The cause of all these surprisingly simple processes, Wolff saw in a special “essential force” (vis essentialis) inherent in living beings and capable of creating a complex organization from formless structures. Wolff's observations did not meet with the proper attention of his contemporaries, and the ironic criticism of Haller and Bonnet deprived them of authority. “Is it not obvious that such an amazingly and harmoniously constructed whole cannot be composed like the parts of a clock or by the accumulation of an infinite number of different molecules; why force our reason to search for mechanical solutions when the indisputable facts themselves lead us to the theory of the pre-existence of germs,” exclaims Bonnet. A purely polemical work by Blumenbach (1781) against preformation had greater success. However, only from the beginning of the 19th century does the development of descriptive embryology begin (Pander, 1817; Purkinje, 1825; Baer, 1827–1828). Baer, with his work “On the History of the Development of Animals” (1828), lays the foundations of modern embryology; he does not link the results of his observations with either preformation or epigenesis—he farsightedly rejects both theories.

Further energetic development of descriptive-embryological research pushes the abstract-theoretical question about the essence of development and the antithesis "preformation-epigenesis" into the background. This ends the first stage of the problem's development. The dispute between the theory of preformation and new formation (epigenesis) is decided in favor of the latter. The revival of the problem at the end of the 19th century occurs on a fundamentally different basis, and the very understanding of Epigenesis changes. A new wave of interest in theorizing is connected with increased attention to the problems of heredity and speciation and with the brilliant successes of cytological research (60s-70s of the 19th century). The lush flourishing of all kinds of theories of vital and hereditary units (Spencer's "physiological unit," 1864; Darwin's "pangenesis" and "gemmules," 1868; Haeckel's "plastidules," 1876; Nägeli's "idioplasm," 1884; de Vries's "pangenes," 1889; Weismann's "germ plasm" and its elements, 1892; Haacke's "gemmarii," 1893, and many others) again puts the problem of Epigenesis and preformation on the agenda. The understanding of these problems differs fundamentally from the Epigenesis of Wolff and the preformation of Bonnet. No one, it goes without saying, disputes the fact of real development and transformation in the embryonic process. The subject of the dispute is now the role of external factors in development. Characteristic of this period is the inclusion of facts from the field of post-embryonic development, heredity, and even speciation as evidence. Therefore, the connection of the Epigenesis of this period with the problems of modifications, inheritance of acquired characteristics, "change" of species, etc., becomes understandable. Instead of the traditional vitalistic Epigenesis, a Lamarckian Epigenesis is born. A more in-depth reflection of this new understanding of Epigenesis is realized in the experimental-embryological line of research flourishing by the end of the 19th century. The attempts of Roux and Weismann to create a preformationist theory of development are refuted by in-depth experimental-embryological analysis in the works of Driesch, O. Hertwig, and others (see Preformation). The epigenetic theories of the latter introduce a new, deeper understanding of Epigenesis and the role of external factors. They speak no longer only or so much about influences external to the entire organism or embryo as a whole, but about the mutual influence of the parts of the developing system. It is a question of the Epigenesis of individual parts, their development under the influence of other parts, which are "external" in relation to them. The specificity of the development of individual parts turns out to be to a large extent a "function of position" (Driesch). However, as is known, even this in-depth understanding of Epigenesis did not save it from the traditional vitalistic appendage (Driesch's "entelechy"); moreover, the brilliant "victory" of Epigenesis in many respects contributed to the revival of vitalism in the late 19th and early 20th centuries. Modern understanding of the problem of Epigenesis. Until very recently, biology was characterized by complete inconsistency and lack of thought regarding the theoretical foundations of the problem in various disciplines. While in embryology, both descriptive and experimental, after the triumph of the epigenetic theory of development of Driesch and Hertwig, the doctrine of the epigenetic nature of development became "school" (official) dogma, in genetics, which developed on the basis of the victorious refutation of Lamarckism, brilliantly anticipated by Weismann, there is a definite inclination toward preformationist ideas. As is known, both understandings in these sciences rely on completely indisputable, firmly established facts; the divergence in interpretation is explained exclusively by incorrect theoretical conclusions, and often by an arbitrary interpretation of the concepts of preformation and Epigenesis. This was partially shown even by W. Roux in his theoretical analysis of the problem (see Preformation), which is usually ignored. What does modern embryology understand by Epigenesis? First of all, it refers to the enormous role of environmental factors in relation to the organism as a whole. Exemplary works of experimental embryologists have placed beyond any doubt the influence of gravity, pressure, temperature, physical-chemical environment, etc., on the process of development (see Developmental Mechanics). It would be erroneous to interpret the "disturbances" of normal development resulting from this in the sense that in the "norm" the role of external factors is less. If we succeed in experimental conditions in achieving a distinct effect only with a more or less sharp disturbance of development conditions, this does not at all prove the absence of strictly specific external factors ensuring normal development. However, on the other hand, it would be elementary theoretical naivety to speak therefore of external factors as the main and only cause of development, to speak, as is often done, of ectogenesis. A necessary specific condition for development to no less, and even to a greater extent, is the biological system and its features. More profound in character are the ideas which consider the action of external factors on a developing system not in the sense of direct immediate determination of it, but through the mediation of special "gradient systems" (Child). It is believed that development is determined by differences in the activity of different parts of the developing system. Activity can be determined in the most varied ways: in the oriented arrangement of any specific substances (chemodifferentiation), or substances providing an excess of energy resources (yolk, etc.), or in the already existing difference in oxidative processes of different sections of the system. Such vectors of activity are called gradients. A whole series of experiments has proven that some gradients can be caused by external factors or that already existing gradients can change their localization and direction under the influence of such factors. Thus, in the developing eggs of most species, an "axial gradient" is discovered very early, i.e., the anterior and posterior, and sometimes the right and left parts of the future embryo turn out to be determined. However, a number of experiments have proven that the concrete spatial localization of such a gradient can be determined by external factors. Thus, in the eggs of the alga Fucus, the determination of polarity depends on the direction of light, the concentration of oxygen, etc., factors, namely, the axis of development in the egg passes in the direction of light, toward the greatest accumulation of oxygen, etc. In amphibian eggs, the plane of the bilateral axis is determined by the place of sperm penetration. But it goes without saying, in all such cases, no matter how one changes the direction of the gradient, the very character of the latter turns out to be predetermined—in the eggs of bilaterally symmetrical species, a similar embryo will develop, while in the eggs of radial symmetry or asymmetrical ones, an differently oriented embryo develops. Thus, the "epigenetic nature" of some gradients (or their direction) does not introduce anything fundamentally new compared to the usual necessary participation of external factors in development. At the same time, this relatively small number of "epigenetically" determined gradients is opposed by a huge system of "preformed" gradients (see Promorphology). The most essential factor of Epigenesis in development is considered to be the mutual influence of the parts of the embryo. The doctrine of the mutual influence of the parts of the embryo received its completion in the modern theory of organizers (Spemann; see Developmental Mechanics). A number of regions of the embryo have been established which determine the character of the development of adjacent tissues. Thus, each "organizer" has a strictly defined "field" ("organizational field") of influence, of "induction." Consequently, the tissues of the "organizational field" develop under the influence of factors external to them. Such cases of "dependent differentiation" in embryology are usually considered decisive proof of the epigenetic character of development. In connection with this, many embryologists generally believe that the processes of determination are limited to the dependence of the development of one part of the embryo on another. From this, the conclusion is already drawn that any "determination" is a purely epigenetic process. At the basis of these reasonings lies a theoretically completely vicious, formal principle: they consider it possible to solve the question of Epigenesis and determination for each part separately. They believe that if the factor influencing a certain part is external to it, then development is epigenetic. However, the "external," "exogenous" character of such an organizer or "dominating region" can be admitted only purely formally. From the point of view of the biological system as a whole, such an "epigenesis" of its individual part is preformed; and again, as in the principle of gradients, it is completely immaterial that the concrete spatial localization of the "organizational field" is conditional and can be changed from the outside. In general, the question of Epigenesis and preformation can be correctly solved only for the entire biological system as a whole, in this case—the developing embryo. Moreover, there is a whole series of facts proving that the basic polarity and differentiation of the maturing egg are determined by the genotype and phenotype of the maternal organism.

Formally speaking, the environment of egg maturation within the maternal organism is a factor "external" to the egg; thus, from the point of view adopted by the majority of embryologists, it should be asserted that the formation of promorphological structures in the egg is an epigenetic process. From the above, it is evident that the question of developmental factors, in particular the role of "epigenetic" and "preformational" processes, must be resolved not only for a specific embryo as a whole, but the species and genetic (successive) specificity of its ontogenesis in general must also be taken into account. In light of such a formulation of the question, one should consider erroneous not only the assertion of the exclusive epigenetical nature of development, but in general, the very antithesis of "preformation-epigenesis" must be considered formal, mechanistic, and not corresponding to the entire specificity of the complex biological laws of ontogenesis.