Two historical schools: preformationism and epigenesis
Since antiquity, the history of embryology has been built upon the opposition of two developmental concepts:
- Preformationism. Proponents of this doctrine believed that an organism develops solely through the growth of a pre-existing microscopic embryo. The school was divided into two camps that debated the localization of the embryo:
- Ovists claimed that it resided within the ovum.
- Animalculists considered the male germ cell to be the vessel. They hypothesized the presence of a tiny human—a homunculus (Homunculus)—inside the spermatozoon. A famous illustration by N. Hartsoeker (1694) depicts such a preformed child inside the head of a sperm cell.
- Epigenesis. The opposing view, according to which new, heterogeneous parts of the organism are formed anew from initially homogeneous material under the influence of external factors. Development here is understood as the sequential creation of new structures.
Interestingly, even Aristotle (4th century BC) considered both hypotheses, but ultimately rejected preformationism and leaned toward the ideas of epigenesis.
Resolution of the debate and early experimental embryology
The epigenetic concept was finally substantiated experimentally by Caspar Friedrich Wolff in his dissertation "Theoria Generationis" (1759). He also proved that embryonic development requires the participation of germ cells from both sexes.
The debate was definitively settled by the cell theory of M. Schleiden and T. Schwann (1838–1839), and it was later established that the spermatozoon and the ovum are separate, highly specialized cells.
Concurrently, the experimental foundation was developing. Thus, Wilhelm Roux and Hans Driesch, conducting experiments on sea urchins, formulated an important principle: embryonic development depends on the interrelationships between cells.
Emergence of scientific and evolutionary embryology
A huge contribution to turning embryology into a rigorous science was made by the Russian academician Karl Ernst von Baer. He systematized the accumulated material, described the formation of two germ layers, the notochord, and other structures. His main achievement was the law of embryonic similarity (Baer's law), which states that development proceeds from the general to the specific. First, features of the phylum appear in embryogenesis, then of the class, order, family, and last of all, generic and specific traits are formed.
The development of the evolutionary branch is associated with the names of A. O. Kovalevsky and E. Haeckel:
- Aleksandr Onufrievich Kovalevsky established a unified developmental plan for multicellular animals (from coelenterates to amphioxus) and substantiated the germ layer theory.
- Building upon his work, Ernst Haeckel formulated the Fundamental Biogenetic Law: ontogeny (individual development) is a brief recapitulation of phylogeny (historical development of the species).
According to Haeckel, one can observe palingenesis in development—ancestral traits (such as gill slits or a notochord in mammals)—and cenogenesis—evolutionarily novel traits (such as provisional extraembryonic membranes in fish, birds, and mammals).
Later, P. G. Svetlov supplemented this knowledge by developing the theory of critical periods in embryogenesis.
Briefly: answers to topical questions
What were the methodology and essence of the experiments conducted by Wilhelm Roux and Hans Driesch on sea urchins?
In early experimental embryology, the methodology of researchers Wilhelm Roux and Hans Driesch involved conducting experiments directly on sea urchins. The essence of their experiments led the scientists to formulate a fundamental principle: embryonic development depends (at least in part) on the interrelationships that arise between the cells of the embryo.
What specific structures did Karl Ernst von Baer first describe in embryos?
Karl Ernst von Baer described the formation of two germ layers, as well as the notochord. The source also indicates that he described "other structures," but they are not specified. In addition, Karl Ernst von Baer systematized empirical data and turned embryology into a science.
Class and exam questions
What is the essence of the Fundamental Biogenetic Law?
Ontogeny is a brief recapitulation of phylogeny. This means that an organism, during its individual development, passes through stages characteristic of its evolutionary ancestors.
What is the difference between ovists and animalculists?
Both schools belonged to preformationism. The difference lay in the localization of the microscopic embryo: ovists believed it was located in the ovum, while animalculists believed it was in the spermatozoon (homunculus).
What discovery finally settled the debate between preformationists and epigeneticists?
The debate was resolved in favor of epigenesis thanks to the establishment of Schleiden and Schwann's cell theory, as well as the work of C. F. Wolff.
What does Karl Ernst von Baer's law state?
The law of embryonic resemblance states that embryonic development proceeds from the general to the specific: first, the features of the phylum are laid down, followed by the class, the order, and finally, the features of a specific species.
What else to study on this topic
- P. G. Svetlov's theory of critical periods
- Mechanisms of cell interactions in the experiments of Roux and Driesch
- Palingenesis and cenogenesis: examples in various animal classes
- Structure and functions of extraembryonic (provisional) organs
- A. O. Kovalevsky's germ layer theory
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