Epigenesis

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Epigenesis or epigenesis ( ancient Greek ἐπιγένησις epigénēsis , German 'subsequent emergence' ) is a term for the formation of new structures from unformed matter during the development of a living being . It was introduced in the 18th century by Caspar Friedrich Wolff ( Theoria generationis , 1759) when dealing with the prevailing preformation theory at the time . Similar views can be traced back to antiquity ( Aristotle ).

prehistory

Aristotle examined the development of the chick in the egg and observed that the yolk is initially still unformed matter and then the different parts of the chick gradually develop. He thought of matter and form as interacting causes : the material cause ( causa materialis ) and the form cause ( causa formalis ). According to Aristotle, a new organism arises from the union of the liquid "semen" of both parents, whereby he saw the female semen in the menstrual blood . He identified this as the material cause, the male as the form cause. The latter creates the new living being from the former. In addition, there is the effect of the soul , which every living being - in contrast to dead bodies - has from the beginning and which controls its development.

The Aristotelian view of the successive development of the embryo from an initially unformed mass was adopted by Christianity and carried on into modern times . However, according to the traditional Christian view (as in Augustine and Thomas Aquinas ), the soul only connects with the developing embryo after some time, in humans around the 40th day of pregnancy. The mixture of liquids, which was previously only animal, only becomes human when it is animated.

A systematic study of the development of chickens was first presented by Ulisse Aldrovandi in his Ornithologia (1599). In it he confirmed the representation of Aristotle. In his Exercitationes de generatione animalium (Exercises on the Generation of Animals, 1651) , William Harvey described with a hitherto unknown accuracy, also in the chicken, how the later organs gradually emerge from a homogeneous starting substance ( primordium ). He postulated an omnipresent formative principle, a “divine architect”, who brought about this process.

Appearance of the preformation theory

Drawings of human sperm, Nicolas Hartsoeker (1695)

In 1677, Antoni van Leeuwenhoek published in the context of his sensational investigations with the microscope - Leeuwenhoek was known for by far the best microscopes of his time - drawings of tiny "seed animals" ( spermatozoa ), which he had discovered in the sperm of various mammals, together with drawings of corresponding ones "Little animals" ( animalculi ) made from human sperm, which the medical student Johan Ham had made. Ham's imaginative drawings, in which the sperm appeared like preformed “little humans”, sparked a debate in the course of which the previous ideas, dating back to Aristotle, were replaced in science by preformation theory towards the end of the 17th century. As a result, the only argument left was whether the pre-formed germ could be seen in the sperm, as the animalkulists claimed, or in the egg instead . Charles Bonnet's proof in 1740 that female aphids can reproduce without males (virgin generation or parthenogenesis ) was considered excellent evidence of this ovistic thesis .

Globular algae ( Volvox aureus )

In addition to the persuasiveness of such alleged evidence for the preformation, there was the fact that the spontaneous generation , the spontaneous formation of living beings from dead material, had become very dubious in the late 17th century due to the experiments of Francesco Redi and others. Accordingly, what Harvey had claimed was no longer believed to be possible, and his work was dismissed as unscientific. Behind this was the then still general belief in the Old Testament creation story and the conviction that the origin (creation) of living beings was not understandable for humans. Now it was assumed that God created already in the creation of the world with all previous and future generations of living beings and that all generations into each nested be and only unfold would. A particularly obvious proof of this was the spherical alga Volvox , in which several generations can be nested within one another.

Problems and controversies

A big problem with preformation theory was that the offspring could not be nested with both parents at the same time. It has also been known since ancient times that mules have the characteristics of both parents. Using this example and that of the mulattos , Pierre Louis Moreau de Maupertuis was the first to criticize this doctrine in 1744. He emphasized that both parents contribute equally to the characteristics of the offspring, and that both the animalkulists and the ovists were therefore in error, and returned to the doctrine of the two seminal fluids. In 1749 René-Antoine Ferchault de Réaumur published a work on the inheritance of six-fingeredness in humans, in which he came to the conclusion that this deformity could come from both the father and the mother. He noted that “these facts are obviously unfavorable for the preexistence of the systems.” Joseph Gottlieb Kölreuter (1761) also found in cross-breeding experiments with different types of tobacco that the offspring had characteristics of both parents. Another problem was the ability of some animals to regenerate , such as the regrowth of a lost tail in lizards or the formation of lost claws in crabs . And as a sensation, the report by Abraham Trembleys (1744) was received, who cut freshwater polyps ( hydra ) into many parts and saw a new polyp emerge from each part.

Nevertheless, the preformation theory remained largely unchallenged in the 18th century. Even when Caspar Friedrich Wolff again presented detailed embryological studies in 1759 and 1768/69, with which he clearly demonstrated epigenesis from today's perspective, this did not convince his contemporaries. It was not until Johann Wolfgang von Goethe's attempt to explain the metamorphosis of plants (1790), in which he described the successive formation of differently shaped leaves in annual flowering plants , that there was a generally positive reception. In 1796 Goethe coined the term morphology , which he defined as the doctrine of metamorphosis: "The shape is a moving, becoming, passing."

In the area of ​​zoology, it was not until the 19th century that the embryologists Christian Heinrich Pander (1817) and Karl Ernst von Baer (1828) succeeded in gaining general recognition for the epigenetic character of embryonic development and thus overcoming preformist ideas.

See also

  • Epigenetics : a branch of research focused on the factors determining the activity of a gene . The adjective “epigenetic” can refer to both nouns depending on the context.

literature

Web links

Wiktionary: Epigenesis  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ A b Jane Maienschein: Epigenesis and Preformationism. In: Stanford Encyclopedia of Philosophy . 2008.
  2. ↑ Ilse Jahn , Rolf Löther, Konrad Senglaub (eds.): History of Biology. 2., through Edition. VEB Fischer, Jena 1985, DNB 850359589 , p. 217 f.
  3. ↑ JWJ Lammers: Johan Ham, de ontdekker van de zaaddiertjes. In: Nederlands Tijdschrift voor Geneeskunde. 118 (1974), ISSN  0028-2162 , pp. 784-788.
  4. ↑ Ilse Jahn (Ed.): History of Biology. 3., rework. and exp. Edition. G. Fischer, Jena a. a. 1998, ISBN 3-437-35010-2 ; Nikol, Hamburg 2004, ISBN 3-937872-01-9 , pp. 210-213 and 256 respectively; also digital: history of biology. Theories, methods, institutions, short biographies (= digital library. 138). Direktmedia Publ., Berlin 2006, ISBN 3-89853-538-X (direct access to the publication - only on the DNB reading room computers - possible via the ISBN link).
  5. ↑ Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. 2nd Edition. VEB Fischer, Jena 1985, p. 231.
  6. ↑ Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. 2nd Edition. VEB Fischer, Jena 1985, p. 232 f. and 236.
  7. ↑ Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. 2nd Edition. VEB Fischer, Jena 1985, p. 244 f.
  8. ↑ Ilse Jahn (Ed.): History of Biology. 3., rework. and exp. Edition. G. Fischer, Jena a. a. 1998, ISBN 3-437-35010-2 ; Nikol, Hamburg 2004, ISBN 3-937872-01-9 , each p. 278 f.