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?
Read MoreHow 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)
Read MoreA 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.)
Read MoreHow long can we sustain the unsustainable?
The rapid increase in the world's population as a whole, the presence of many people that live below the level of any equitable living standards, the emergence of large and rapidly developing economies such as India's and China's, and the profligate model of the American lifestyle (and to a slightly lesser extent, the Western European) have all contributed to concern over the "carrying capacity" of the planet. How many people can obtain adequate amounts of clean water, food, and shelter, let alone automobiles, computers, and a constant supply of consumer goods? The environmental group Bioregional.com calculates that if everyone on earth adopted the British lifestyle, it would require three planets to support. The American lifestyle would require seven planets, but we still only have one. Will wars and catastrophes be the only way to know when those limits have been reached?
The growing awareness of environmental consequence, and the sense that "we can't go on this way" without serious adverse consequences for future generations underlies the development of the term sustainability. The expression was first put into widespread political use by the United Nation's World Commission on the Environment and Development, which issued a report in 1987 known as the Brundtland Report, named after its chairperson, Gro Harlem Brundtand, the former prime minister of Norway.
Although it draws on scientific study and environmental forecasts, the concept of sustainability is a political and moral term, like justice or freedom, and the history of sustainability as as much social, political, and economic as it is environmental.
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