The larval ascidian nervous system: the chordate brain from its small beginnings

The body plan of the tadpole larva of ascidians, or sea-squirts, is widely presumed to be close to th;tt of the hypothetical ancestor of all chordate animal groups, including vertebrates. This is nowhere more obvious than in the organization and development of the dorsal tubular nervous system. Seve ral recent developments advocate this model neural system for studies on ne urobiology and neurogenesis. These include advances in our understanding of development ire ascidian embryos and of differentiation among the cellular progeny of its neural plate; the application of transgenic and mutant appr oaches to studies on ascidian larval neurones; and the prospect of advances in genomic analyses. In addition to providing ways to study a working chor date brain in miniature, all these offer insights into the ancestral condit ion of the developing vertebrate brain.