Abstract

98. r 1998 John Wiley & Sons, Inc. Introduction The central nervous system (CNS) of the vertebrates is very different from that of invertebrates such as insects. The former acquire, from dorsal tissue, a closed neural tube, while the latter possess ventral alignments of ganglia, the neural cord. Yet it is becoming clear that development of these structures results in both cases from the action of universal mechanisms. The proteins involved, such as transcription factors, long-range morphogens, membrane receptors, and their ligands, are homologous. The spatial distributions of the associated genetic activities reflect the overall inversion from ventral to dorsal. (1) This is quite satisfying, of course, but begs the next question: how do these homologous genes and products lead to such different phenotypes, namely, isolated ganglions in one case, and a continuous, closed tube in the other? The construction of a theoretical framework w

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