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freshwater hydra, mosaic development, vegetative propagation, gastrulation, gonads
Development (biology), branch of biology concerned with describing and understanding how a fertilized egg or spore or bud turns into an adult organism. More inclusive than embryology, the term also encompasses such processes as regeneration of limbs in many animals and vegetative propagation as found in higher plants. In addition, biologists are interested in the relationship between the processes of development and those of aging.
Sexual reproduction requires a single-cell stage. If large multicellular size has adaptive advantages, then the life cycle must necessarily include a period of development from the single cell to the mature form. The process of development has three components: growth (size increase), morphogenetic movement (the shaping of patterns and forms), and differentiation (the change from undifferentiated to specialized structures).
Formative cell movements may occur with or without growth. When cells move and grow simultaneously, the process is called morphogenesis. Morphogenetic movements are the rule in multicellular animals, but they are generally absent in plants because of the hard cell wall. In the development of a vertebrate, the first important morphogenetic movement is gastrulation, a cell movement that can occur in a number of ways but invariably results in an embryo with two cell layers formed from one. Subsequent morphogenetic movements are numerous, such as the aggregation of cells to form limb buds or the migration of the primordian germ (sex) cells to the region of the gonads (testes or ovaries).
A striking phenomenon of cell differentiation that occurs commonly in plants and in many animals is the replacement of lost parts by regeneration, the rebuilding of differentiated tissues. In general, more complex animals have fewer powers of regeneration than the simple forms that appeared earlier in the evolutionary chain. Cnidarians such as the freshwater hydra show spectacular power to regenerate all parts of the body; fish and amphibians can often regenerate fins or limbs; mammals can regenerate the liver and the blood cells (the latter normally being in a constant state of differentiation and destruction).
Regulative and Mosaic Development
An undifferentiated mass of embryonic cells can also be divided, and a complete organism will grow from each separated portion. This process is called regulation or regulative development. In a famous experiment at the close of the 19th century, the German embryologist Hans Driesch cut a very early sea urchin embryo longitudinally, and each half embryo produced a dwarf but normal larva. At about the same time two American embryologists, Edmund Beecher Wilson at Columbia University and Edwin Grant Conklin at Princeton University, showed that in mollusks, worms, and sea squirts such an operation produced two abnormal half embryos. They called this mosaic development, and they contrasted it to Driesch's regulative development. The differentiation process seems to start earlier in mosaic eggs than in regulative eggs; mosaic eggs are said to be determined relatively early.
Control of Growth
Understanding the molecular machinery within cells gives biologists a direct basis for understanding growth, because growth is the synthesis of new protoplasm, and biologists know the basic mechanism of this synthesis. One key gap, however, exists in this knowledge. Biologists want to know not only how substances are synthesized but also how growth is controlled so that the proportions of an animal or plant remain consistent from generation to generation. The direction and amount of growth, which are responsible for shape and size, clearly are also genetically controlled. The way in which this control is exerted, however, is an active field of research involving the study of chemical messengers that stimulate and inhibit cell division and that are asymmetrically distributed so as to control the direction of growth.
Bonner, John Tyler, M.A., Ph.D., D.Sc.
George M. Moffett Professor of Biology, Princeton University. Author of ''Cells and Societies'', ''On Development: The Biology of Form'', and other books.
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