Resources for preformation Time Traveler! Explore the year a word first appeared. Dictionary Entries near preformation preform preformant preformat preformation preformative preformed wire rope preformer. Statistics for preformation Look-up Popularity. Time Traveler for preformation The first known use of preformation was in See more words from the same year. Comments on preformation What made you want to look up preformation? Get Word of the Day daily email! Need even more definitions? Words at Play Is it 'Grifter' or 'Grafter'?
Ask the Editors Word of the Year: Literally How to use a word that literally drives some people nuts. In , American biologist William Morton Wheeler suggested that there are just two different kinds of thinkers. Some see change and process, while others see stability. Heraclitus, Aristotle, physiology, and epigenesis characterize one way of looking at the world, while Parmenides, Plato, morphology, and preformationism characterizes another.
These are, Wheeler felt, stable and persistent classes, with just the nature and details of their differences have changed over time. Yet by the end of the 19 th century, he argued, neither a strict preformationist nor a strict epigeneticist who ignored new evidence and new reasoning should prevail. Rather he called for a middle ground, for: Wheeler was stimulated by recent late 19th century debates, themselves provoked by a flood of new discoveries. August Weismann and Oscar Hertwig provided particularly strong and contrasting positions.
Weismann had begun from an epigenetic viewpoint and initially rejected the idea that individual form emerges through the unfolding, or evolution, of pre-existent form in the inherited germ. But by the time his Das Keimplasm appeared in translated into English in , Weismann had changed his mind. The key is in the special material of the germ cells, Weismann decided. Within these cells lies all the determinants necessary to direct development.
Inheritance, that is, causes development and differentiation. Weismann's theory postulated the existence of several levels of hypothetical units. By the s, it was agreed that individuals begin as cells, those cells contain nuclei, and that nuclei contain chromosomes. The chromosomes are the material of heredity, Weismann postulated, and they consist of a string of determinants, correlated with characters in the organism. Each determinant consisted of a number of material particles called biophores, inherited from both parents. These biophores compete with each other and some prevail, which then determines the character of the determinant, which in turn determines the character of the organism.
During each cell division, the original whole chromosomal material is divided up, so that the effect is like a mosaic.
Each cell becomes the right type just because of the action of the determinants distributed to it. Conditions external to the cell itself cannot guide development, but rather the causes lie within. And cell differentiations that make up complex organisms are predetermined.
pre·for·ma·tion
He felt that Weismann made too many assumptions and that he actually provided no real explanation of development and differentiation at all. In contrast to Weismann's preformationism, Hertwig pointed to the interactions of cells and to the differences among cells for the source of differentiation.
The complexity is not built in from the beginning, but emerges over time, dynamically, and interactively. A cytologist himself, Hertwig saw the intricate structures already part of the unfertilized egg, and the changes that occur with fertilization. The egg is not a completely unstructured blob, but rather a complex of different materials that can respond to influences both within the egg and from the external environment. Cells behave like small organisms, and it is the interactions of these separate organisms that makes the whole.
Epigenesis and Preformationism (Stanford Encyclopedia of Philosophy)
As Hertwig put it: I cannot regard the development of any creature as a mosaic work. I hold that all the parts develop in connection with each other, the development of each part always being dependent upon the development of the whole.
- Navigation menu.
- The Sweet Taste of Revenge?
- preformation;
- preformations - Wiktionary!
Hertwig and Weismann continued to argue both about the metaphysical nature of the organism as well as about the epistemological demands for gaining knowledge about it, as did others, with no generally accepted way to resolve the issues. Given the information at hand, it seemed that Wheeler was right. There were just two different types of people, drawing on two different sets of values and emphases.
Both relied on assumptions, and only more evidence could help move the discussion forward. Wilhelm Roux adopted much the same approach as Weismann's and so, at first, did Hans Driesch. Yet their experiments ultimately led to new approaches and revised interpretations of what was at issue with epigenesist and preformationist accounts of development. In , Roux published results of his experiments on frog eggs.
Working on the assumption of a mosaic type preformationism, Roux was persuaded that starting from the very first cell division, each cell would be different because it was already predetermined to be different. Roux proposed an experiment, a simple and elegant experiment on the face of it.
He proposed to take a developing frog egg, after the first cell division, and to separate the two cells.
Epigenesis and Preformationism
Finding it impossible to separate the two cells, however, he simply killed one by inserting a hot needle. That cell just hung there like a blob of material and no longer differentiated. The other half organism, or single cell proceeded to develop, in Roux's interpretation, as it normally would have developed [Roux ].
The half became a half, just as it should if it were already preformed or predetermined as to its fate in the organism. Roux had, it seemed, confirmed the mosaic hypothesis. A few years later, Driesch was working at Naples and had access to sea urchin eggs. Fortunately, because of Oscar and his brother Richard Hertwig's study of these eggs, Driesch knew that if he shook the two cells they would separate completely. Driesch reported agreeing with Roux and intending to confirm Roux's results. But since the sea urchin eggs could actually be separated, he felt that his results would be even more convincing.
Imagine his surprise when he looked the next morning after separating the eggs and found not two half embryos but two smaller sized urchin larvae. I thought the formations would probably die. In later experiments they developed even further, into apparently perfectly normal pluteus larvae, and even the four cell stage could do the same. Driesch concluded with an epigenetic account, but an epigenesis relying strictly on materialistic factors at least that was his initial response; Driesch did turn later to a version of vitalism. The early embryo retains its totipotency, he concluded.
The fertilized egg clearly has the capacity to become a whole organism and so, apparently, do the cells after the early cell divisions.
preformations
Not a mosaic of cells already predetermined by their inherited determinants in the nucleus, the early embryonic stages are instead a population of separate totipotent organisms, each capable of becoming a whole. It is only the interactions among them under normal conditions that lead to a complex, organized, integrated differentiated organism. It might seem that Roux would have to acknowledge the superiority of Driesch's approach, since he had actually separated the cells.
Instead Roux countered with an additional hypothesis. The nucleus retains the capacity to adapt, especially in simpler organisms. They need the capacity to regenerate when injured, and therefore the mosaic determination simply has not occurred yet. It seems that Wheeler was right.
- ARVO 2011 Annual Meeting - Section IM;
- preformation - Dictionary Definition : theranchhands.com?
- John F. Kennedy und die Wahl zum amerikanischen Präsidenten von 1960 (German Edition).
- Preformationism.
- “Bunker Busters”: Robust Nuclear Earth Penetrator Issues, FY2005-FY2007.
- Fast Neutrons and High-LET Particles in Cancer Therapy: Clinical Status (Recent Results in Cancer Research);
- Epigenesis/Preformation.
- Preformationism - Wikipedia?
- Letteratura e metafore della realtà. Vol. 2: La critica letteraria. (Scenari della comunicazione) (Italian Edition);
- Quotes of Love: Magical Love Quotes - Timeless Romantic Quotes to Make That Special Man or Woman Fall in Love With You Again…Or For the First Time! (Valentines Day Romance)!
Roux, Weismann, and others had decided that development must be guided by predetermined mosaic differences. Preformation, stability, and predictability stood on one side, with epigenesis, dynamic process, and change on the other. And, as Wheeler noted, by the way forward lay between the extremes of strict preformation or epigenesis. Wheeler's dissertation director Charles Otis Whitman agreed. Whitman felt that what biology needed was a clear statement of the alternative views, and then movement to a new standpoint examining how much depends on the organism's developmental response to external conditions drawing on preformation, rather than on programmed internal unfolding alone.
Whitman, Edmund Beecher Wilson, and others at the Marine Biological Laboratory in Woods Hole, Massachusetts, dedicated considerable energy to discovering the nature of each cell and its internal organization and relationships, in an attempt to discover the relative contributions of preformation and epigenetic development to a materialistic explanation of development. By the early 20 th century, they had moved toward an understanding that included a fertilized egg that was to some extent preorganized and differentiated, including in the nuclear chromosomes, and also a capacity of the individual organism to respond to changes in its environment or to self-regulate.
This was an epigenetic view that allowed some minimal predeterminism. The epigenetic embryologists emphasized the individual's development, but an alternative new form of preformationism soon arose in the form of genetics. This pointed to the nuclear chromosomes as the locus for the causes of differentiation. Yet unlike Weismann and Roux, the new geneticists saw the genetic material not as divided up into a mosaic as an explanation of difference.
Rather, the inherited nuclear material was the same in every cell, but it acted differently according to an internal program. The interpretation appealed to some for metaphysical reasons, since it focused on the material units of heredity and apparently of causation. Epistemologically, it was more difficult to point to the evidence that inherited genes explain development.
This is not the place for a history of genetics, which has been offered many times elsewhere. The important point here is that genetics brought a new form of preformationism. Instead of a dynamically acting organism taking its cues from the environmental conditions and from the way that cells interact with each cell division, the 20 th century brought a dominant and popular view that has often emphasized genes as programmed to carry the information of heredity, which was also the information necessary to construct an individual.
Of course, there have been calls for alternatives or interactionist models where heredity and development, epigenesist and preformation, work together, but these have often been offered as alternatives than as central interpretations. In the beginning of the 20 th century, at first Thomas Hunt Morgan resisted the Mendelian-chromosome theory of inheritance that saw inherited units of heredity carried on chromosomes as determinants of developed characters. If all the cells contain the same chromosomes, then how can their inheritance explain anything, he asked.
Like his Woods Hole colleagues, Morgan did not see how inherited chromosomes could explain development of form from non-form. Biology biology the theory, now discredited, that an individual develops by simple enlargement of a fully differentiated egg cell. Switch to new thesaurus. References in periodicals archive? Mahmoud Mirbagheri said the boost was made possible after completion of 25 acidizing, cementing and preformation operations over the wells operated by the company during the current Iranian calendar year which began on March In this crosslinking system, ethanol is present to block or hinder the reaction between the carbamylated resin and the CHDA, through the preformation of acet als.
Isocyanate-free polyurethane technology for wood coating applications: Approaches to reducing VOC in clearcoat formulations.