Embryology

EMBRYOLOGY, the science of the individual development of organisms. The beginning of E. dates to the 16th century, when the Dutch anatomist Coiter in 1572 first gave a description (very incomplete) of the development of the chicken egg. In the 17th century, significant work in this field was done by Fabricius of Acquapendente (1621) and especially by the famous physiologist and anatomist William Harvey, whose book was published in London in 1651; Harvey applied the comparative method and studied not only the development of the chicken egg but also the embryo of a mammal (roe deer). An essential stage in the development of E. was the work of Caspar Friedrich Wolff, who studied in detail the development of the chick (1754). In 1825 appeared a very detailed study of the development of the chicken embryo by Johann-Evangelist Purkinje. Finally, in 1827, Karl Ernst von Baer publishes the first detailed and fully scientifically substantiated report on the development of the egg of mammals and humans. The subsequent development of E. goes in close connection with the general development of descriptive morphology and reaches by the second half of the 19th century a very significant degree. This powerful development of E. is in direct connection with the improvement of research techniques and in particular with the great successes that the microscope technique had made by this time, ensuring the possibility of observing developmental processes with sufficient completeness. It can be considered that descriptive E. of vertebrates reached its peak by the 1880s. This time marks the emergence of a new, extremely important area of E.-experimental E., or mechanics of development. The development of this area of E. and the dominance of the causal-analytical method is associated with the names of W. Rou, H. Driesch, and others. The new direction in E. can be characterized by the posing of the question "why" rather than "how", which is characteristic of descriptive E. The period of flourishing of experimental E. is associated in recent years with the work of Spemann and his school (Mangold, Holtfreter, and others), devoted to the problem of autonomous and dependent development of organ rudiments and parts of the embryo (the problem of organizational centers). The methodology of embryological research, besides description, comes down to various experiments: this includes the effect of various factors (radiant energy, heat, electricity, chemical effects, etc.); the goal of these experiments is to clarify the possibility of changing the course of development and, first of all, to study the regulations (regulations) of development-a problem of high fundamental interest. The methods of experimental E. are very diverse and come down primarily to marking (Vogt), the essence of which is the staining of areas of the egg with a vital dye and subsequent determination of the fate of a given area. Furthermore, the method of transplants has great importance, when an undifferentiated organ rudiment (usually differing in pigmentation, size of cells or nuclei) is introduced into the embryo and its fate is traced in the host embryo. The defect method-removal of certain areas from the embryo and study of the development of the remaining parts. Furthermore, the method of explantation has great importance-the cultivation outside the embryo of its various areas, making it possible to trace the differentiation of individual organs and tissues, which has exceptional value for histogenetic research. The external effect of various physical and physicochemical factors is studied in their influence on the embryo as a whole and on its individual areas. Finally, in recent years, great importance has been attached to the live observation of the development of eggs of higher vertebrates (birds, mammals). The method that makes it possible to carry out such observations is the removal of part of the shell of a chicken egg and its replacement with a glass window, allowing unhindered observation and cinematography of embryo development; for mammalian eggs (rabbit), a method has been developed for cultivating the embryo in a chicken egg, starting from early stages, making it possible to carry out systematic observation of the development of the embryo of these animals. E. is of essential interest not only in itself but also as a discipline having high importance for all biology, in particular for evolutionary theory. The so-called biogenetic law, first formulated by J. Müller and detailed by E. Haeckel, states, as is known, that ontogeny repeats phylogeny. Thus, the study of embryonic development allows to a certain extent to form an idea of the course of the latter. However, this law* especially in recent years, has been repeatedly criticized, mainly in regard to the weight of cenogenesis, i.e., changes arising in individual life and transmitted by inheritance in the development of subsequent generations. The analysis of the relationships between evolution and E., which has been largely carried out by Academician A. N. Severtsov and his school, shows that in a number of organs sensitive to the effects of the external environment, changes arise, designated by Severtsov as protolaxes (primary changes) along with correlated changes of other organs (deitolaxes). It has been established that traits evolve, on the one hand, due to changes at early stages of development (embryonic variation), on the other-by changes at the final stages of embryogenesis (the method of superimposition or loss of organs). In this case, changes in the organs of the adult animal, progressing and causing the appearance of new traits, shift the initial changes to the stage of embryonic development, turning into embryonic traits. The central problem of E. is the solution of the question of how development occurs-by the type of unfolding of already existing, hidden in the egg traits (preformation) or by development dependent on external influences (epigenesis). The theory of preformation was vigorously defended by Cuvier, while the first epigeneticist is the aforementioned C. F. Wolff. Furthermore, the great importance in establishing epigenetic views was had by the works of Reichert, Bischof (1843) and especially Kölliker (1844) on the development of cephalopod mollusks. However, this dispute cannot be considered resolved to this day, since later in his theory of organ-forming areas of the egg His, to a certain extent, revived the doctrine of preformation. In the study of these questions, the problem of determination of embryonic rudiments arose, which by most researchers is considered in the spirit of the basic ideas of Rou, i.e., the opposition of dependent and independent differentiation (self-differentiation). The solution of this same problem is devoted to the numerous works of the Spemann school mentioned above (the doctrine of organizers).-The next chapter of E. consists of the study of the morphology and physiology of early stages of development (fertilization, cleavage, laying down of embryonic sheets); research in this field is closely associated with the names of Lebedev, Hertwigs, Lillie, and others. Finally, a major division of E. is the problem of histo- and embryogenesis, dealing with the development and formation of individual organs, tissues, and cellular elements. Without understanding E. at the present time, interpretation of facts from the field of normal morphology and physiology is impossible, consideration of data from E. is necessary when solving problems of general and private pathology.-In the system of medical education, E. (usually together with histology) is given a significant place.