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Developmental biologists produce a high yield of mutant individuals by exposing breeding flies or their eggs to x-rays or mutagenic chemicals. When a new individual fails to develop, or when it develops with an obvious abnormality, the research team swoops down to identify which gene has gone awry. Step by painstaking step, as critical developmental genes are identified one by one, scientists are beginning to piece together the puzzle of how life develops. But as old mysteries are solved, new ones quickly arise, as demonstrated by the recent progress researchers have made in the following key areas:

Chemical controls in the egg: The development of every complex organism begins long before the sex act, sometimes months or years before egg and sperm unite. For example, each egg must contain a suite of complex chemical messages to guide an embryo’s initial formation. In flatworms, for instance, the egg’s first cell division always results in a larger cell to the front and a smaller cell to the rear. Even if one of those two cells is removed, the next division again yields a larger and a smaller cell in the same orientation. In this way, in fact, the separation of head from tail occurs right from the start by a chemical signal in the egg. But the egg can’t control development forever. After two cell divisions (four cells total) removal of any one cell will result in grievous deformity in the flatworm. Evidently, from that point on, the cells themselves send each other signals that guide development, but exactly how is the subject of further investigation.

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