Formation of Individuals/Biology/Mayr:
Definition Parthenogenesis: Asexuality: in some organisms, individuals develop themselves from the eggs, fertilization is not necessary. E.g. Aphids, plankton crustaceans: here sexual and asexual generations alternate.
Sexuality: increases diversity and thus defense against diseases.
Pangenesis theory: (old) Theory, according to which each body cell contributes hereditary material. From Aristotle to the 19th century.
PreformationVsEpigenesis (already in Aristotle, then to the nineteenth century).
VsAristoteles: he believed falsely, only female organisms could possess eggs.
Egg: the actual mammal was discovered only by Karl Ernst von Baer in 1827. It was recognized that the ovary is the counterpart to the testis.
DNA: discovered by Johann Friedrich Miescher (19th century).
Definition Preformation: Eggs produce individuals of the same species. Therefore it was concluded that egg or sperm is already a miniature of the future organism.
Logical consequence: in this organism all future offspring must again be contained in a miniature edition! (nesting). Numerous contemporary pictures did show such small miniature humans (homunculi) in the spermatozoon.
Epigenesis: thought that the development came from an entirely unformed mass. "Vis essentialis." Each species has its own peculiar "essential force".
This was completely opposed to the uniform forces described by the physicists, e.g. gravitation.
Definition Epigenesis: (Duden) Development during the life history of the individual, in contrast to ontogeny and phylogenesis (stem history)> Maturana.
Nevertheless, the epigenesis prevails in the controversy. Solution only in the 20th century: difference between Definition genotype (genetic constitution of the individual) and Definition phenotype (totality of perceptible characteristics).
Cell: how does it come that the nerve cell develops so completely differently as a cell of the digestive tract?
Cell division: Wilhelm Roux (1883) concludes the complex internal differentiation of the cell: Solution: particles must be placed on a thread, and this is divided! Confirmed later.
Cell: passes through a differentiation process, only a small part of the genes in the nucleus is active.
Cell development: in taxa with regulatory development (e.g. vertebrate animals) there are no fixed early cell lines, but extensive cell migration. Induction (influence of already existing tissues on the development of other tissues) largely determines the specification of the cells.
Cell migration: pigment and nerve cells make extensive migrations through the organism. Often they follow clear chemical stimuli._____________Explanation of symbols: Roman numerals indicate the source, arabic numerals indicate the page number. The corresponding books are indicated on the right hand side. ((s)…): Comment by the sender of the contribution.
Das ist Biologie Heidelberg 1998