INTRODUCTION Two axial structures, a neural tube and a notochord, are characteristics of chordates and therefore, information about developmental mechanisms of the neural tube and the notochord in lower chordates is essential for an understanding

Two axial structures, a neural tube and a notochord, are characteristics of chordates and therefore, information about developmental mechanisms of the neural tube and the notochord in lower chordates is essential for an understanding of the origin of the chordate body plan (Satoh and Jeffery, 1995). The neural tube of ascidian larvae is composed of about 340 cells, and is divided into three regions along the anteroposterior axis, which are, from anterior to posterior, the sensory vesicle, the visceral ganglion and the caudal neural tube (Nicol and Meinertzhagen, 1991). The sensory vesicle is composed solely of the a-line (anterior-animal) cells (Nishida, 1987). The visceral ganglion present at the junction between the trunk and tail consists of the A-line (anterior-vegetal) cells. The caudal neural tube running along the length of the tail consists of four (dorsal, ventral and two lateral) rows of ependymal cells: the lateral and ventral cells are of A-line origin and the dorsal cells are of b-line (posterior-animal) origin. Beneath the neural tube, a stack of exactly 40 notochord cells runs along the tail. The anterior 32 cells (primary notochord) and the posterior 8 cells (secondary notochord) are derived from A-line and B-line cells, respectively (Nishida, 1987). Cellular interactions that specify the neural tube and notochord of ascidian embryos have been extensively 5597 Development 129, 5597-5608 © 2002 The Company of Biologists Ltd doi:10.1242/dev.00156