morphogenesis

B/Z Reaction

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B/Z Reaction

Chemical reactions do not generally display dynamic patterns or spatial order. The Belousov-Zhabotinsky reaction, discovered in 1951, may be the first completely understandable laboratory example of pattern formation in a chemical system that involves nothing more than chemical reaction and molecular diffusion. That same year, in 1951, Alan Turing investigated the theoretical possibilities of pattern formation by reaction/diffusion as "The chemical basis of Morphogenesis." The B/Z reaction is an example of a chemical system that shows spatial, periodic and wave properties that suggests that morphogenetic self-organization might follow similar pathways in both inorganic and organic systems. (cf. Slime Mold)
from Arthur T. Winfree, When Time Breaks Down, p. 168

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field

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field

jackson pollock number IIa, 1948

In The Evolution of Physics, Einstein described the collapse of the mechanical world-view, leaving an intellectual vacuum before the radically new "field" theory could emerge. A field describes the behaviour of a dynamic system that is extended in space, through kinetics (interaction in time) and relational order (in space). It is a function of space and time coordinates that assigns a value of the field for each of the coordinates.  Jackson Pollock, Number IIA, 1948

In current physics, several kinds of fundamental fields are recognized: the gravitational and electro-magnetic fields and the matter fields of quantum physics. Physicists talk about two kinds of fields: classical fields and quantum fields, although, for the most part, they believe that all fields in nature are quantum fields, and that a classical field is just a large-scale manifestation of a quantum field. 
"A classical field is a kind of tension or stress that can exist in empty space in the absence of matter. It reveals itself by producing forces, which act on material objects that happen to lie in the space the field occupies." (Freeman Dyson, "Field Theory," in From Eros to Gaia, p. 93) 

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embryo

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Development is the process that transforms an egg into a growing embryo and eventually into an adult form.

How is this process to be understood? According to Scott Gilbert, the aesthetic of embryology separates it from other areas of biology. It is an aesthetic informed by the ordered, directional change manifest during the life of individual organisms, as they develop from a single, fertilized egg into complex patterns of diifferent, yet interacting cell, tisssues, and organs.

A few questions have dominated the study of embryology:

First of all, how is the extraordinary process of development regulated? How does a single-celled organism turn into a highly differentiated one with millions or even billions of cells? Do Genes control development? see genotype / phenotype

Is the final form of the organism set from the start? Or are there different paths of development available to the embryo? (For a discussion of preformism and epigenesis, see epigenesis}

What is the relation between the sequence of development and the process of evolution? Why do embryos of different species look so similar, and how do they end up so different?

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induction

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How can a cell "know" to respond to the presence or absence of an enzyme? What is it that induces genes to work only when needed?

Using a fine baby's hair, taken from his own daughter, Hans Spemann tied off and separated the two halves of a two-celled newt embryo. The cells on either side of the knot gave rise to normal newt tadpoles. When Spemann divided the egg differently, by tying it perpendicular to the furrow between the two cells of the embryo, he obtained a dramatically different result. Only one side made a normal tadpole, while the other made a disorganized mass of belly tissue. This eventually lead to the recognition that a region of the embryo, called the dorsal lip of the blastopore, was critical for the organization of the embryo. If this region of the embryo was removed, the embryo formed a blob of tissue lacking structures that normally form on the top (dorsal) side of the animal. In 1924, Spemann proved that a graft could induce host tissues adjacent to it to completely change their fate and to form a second embryo in relation to the graft. If the dorsal lip was transplanted to the presumptive belly region of another developing embryo, it organized a second embryonic axis, and two embryos formed that were joined together. Spemann dubbed this region the "organizer" because he deduced that it organized the dorsal parts of the embryo into neural structures and could induce development of another embryonic axis. All organizers share the property of influencing the formation of pattern, or morphogenesis, in tissues or cells. The basic interpretation of their special activity is that the cells of organizers produce substances that can influence the development of other cells. Such substances have been dubbed morphogens. It has long been thought that morphogens produced in one site diffuse outward and form concentration gradients from their source. The idea then is that cells surrounding the source respond to the amount of morphogen they experience. The affected area is also called the zone of polarizing activity (ZPA). Recent advances in embryology have correlated these zones with the expression of specific genes (toolkit genes)

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morphic fields

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A field is a region of physical influence. Fields are not a form of matter, rather, matter is energy bound within fields. In current physics, several kinds of fundamental fields are recognized: the gravitational and electro-magnetic fields and the matter fields of quantum physics.

The field concept in biology has its origin in the work of Hans Driesch, although the concept itself was elaborated by A. Gurwitsch and P. Weiss. (see account in Gerry Webster and Brian Goodwin, Form and Transformation, pp 94-100) For Joseph Needham, fields are "wholes actively organizing themselves."

In the last decade of the nineteenth century, the embryologist Wilhelm Roux proposed a "developmental mechanics" (Entwicklungsmechanik ) to account for origin and maintenance of organisms through a causal morphology that would reduce them to a "movement of parts," and would prove that biology and physics were completely one with each other. Roux sought to transform biology from a purely historical into a causal discipline through analytic thought and experiment. His "mosaic theory" described development as the self-differentiation of hereditary potentialities with the irreversible functional differentiation among cells. This hypothesis was supported in part by Roux's own experiments at the marine biological station in Naples. When he killed one of the first two cleavage cells in a frog's egg, the surviving cell, as he expected, gave rise to only half of a normal embryo.

in 1891, while working at the Naples station with a different organism, Hans Driesch obtained radically different results. Driesch demonstrated that, contrary to the Roux-Weismann hypothesis, each cell of a sea urchin embryo, when isolated at the two-cell stage, does not produce a half-embryo but a complete, miniature pluteus larva of normal form. (see mechanism / vitalism for philosophical interpretations of these experiments.)

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morphology

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morphology

Morphology is an "account of form," an account that allows us a rational grasp of the morphe by making internal and external relations intelligible. It seeks to be a general theory of the formative powers of organic structure. 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)

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