Morphogenesis

Morphogenesis is the process by which the phenotype develops in time under the direction of the genotype.
The explanation of morphogenesis requires a theory of the gene as well as theories for those properties of the organism revealed by experimental embryology and experimental morphology.

One of the most commonplace and reproducible instances of morphogenesis is the development of form in multicellular organisms, between fertilization and death. (see embryo.)

Historically, the question of the genesis of form was disputed between preformism and epigenesis. The Pre-Darwinian project of rational morphology was to discover the "laws of form," some inherent necessity in the laws which governed morphological process. It sought to construct what was typical in the varieties of form into a system which should not be merely historically determined, but which should be intelligible from a higher and more rational standpoint. (Hans Driesch, 1914, p. 149)

In his classic 1917 book On Growth and Form, D'Arcy Thompson analyzed biological form in terms of physical forces such as viscosity or gravity. Biological form and engineering design were directly related as conforming to or resisting physical force. "The heel of our shoe may wear down with use, but the callouses on our foot grow with use." In a famous set of diagrams, he showed family ressemblences between species of fish by deforming grids through smooth coordinate transformation, suggesting that topology is basic to the overall plan of an organism, although he did not propose a set of forces that might produce these morphological differences. (see mapping ) Nor did he account for the basic topology of an organism.

Today, the discussion is between the mechanisms by which development is realized: between the differing viewpoints of the "atomistic" view, which sees parts of the organism as a "mosaic" of building blocks, with their simple rules of interaction implying the whole, and the " field view"(or structuralist tradition) of self-organization, in which the whole is defined by invariant relations, and the parts emerge as the result of changes that conserve those relations. (These two views derive much of their respective inspirations from the particle and field (e.g. wave) metaphors of physics) (see Yates. p. 131)

In modern theories of morphogenesis, the Weismannist tradition of separating the "active" germ plasm from the "passive" somatoplasm, has resulted in the disappearance of the organism as an organizing totality. Still, the relative importance of genetic information, of "organizers," or of the "competence" of tissue to differentiate are far from resolved in experimental embryology. (see morphic fields )

Are there "hidden" structures which must exist if morphogenesis is to be rationally explained? How do undifferentiated cells know to follow the correct spatiotemporal path to follow to take their proper place in the adult organism? As Michael Polanyi points out, the study of morphogenesis starts with a definite concept of "success, or "rightness." There is an anterior knowledge of the system's total performance and a concept of "true" and typical shapes.

While no one knows how the fate of an individual cell is decided, it is clear that neighboring cells interact and that there are chemical gradients within the growing embryo. (see Alan Garfinkel "Slime Mold") One of the first stages in the morphogenesis of the embryo is the formation of a gradient in the system.

In the 1950's, Alan Turing proposed a mathematical F= "metaphor___model.html#61"> model involving the reaction and diffusion of unspecified chemical compounds that he termed morphogens. Turing wondered how it was possible for a symmetrical, fertilized egg to develop into an asymmetrical animal. He postulated that a system of chemicals that could react and diffuse would under certain conditions break the symmetry of the system. Turing's model for morphogenesis had a large impact when the Belousov-Zhabotinsky reaction was discovered.

Contemporary developmental biology believes that the fate of each cell is determined by the concentrations of various morphogens, and that these concentrations are determined by dynamical systems that display positive and negative feedback, and which describe how morphogens interact and change their levels and types over time.

Stuart Kauffman's hypothesis is that Morphogenesis may be inherently robust. When a variety of different developmental mechanisms are integrated into a compound mechanism, this integration will lead to simplicity rather than complexity. The specific form of symmetry breaking currently established in each submechanism will choose or determine the ways in which symmetry is broken in another mechanism (p. 637) This results in families of forms that are the probable outcome of of the mechanism. Kauffman believes that their basins of attraction occupy large areas of the state and parameter space. (see morphospace )

René Thom developed catastrophe theory as a mathematical way of addressing the work on morphogenesis done by C.H. Waddington in the 1950's. Since the "noble death" of catastrophe theory, Thom has tried to reduce the scope of expectation that it ellicited. He points out that the theory deals with qualities not quantitites and is not meant to be predictive.

As currently conceived in A-life studies, the process of morphogenesis would be the result of a large number of non-linear and local interactions.

What relation does morphogenesis have to the creative process in art? John Dewey used the simile to describe the growth of a work from a faint suggestion in the mind of the author to the finished composition, likened to the stages in the growth of a living being from the germ to the embryo to the fully developed organism. It is almost impossible to avoid the use some organic metaphor in describing this process.