DEVELOPMENT OF THE CENTRAL NERVOUS-SYSTEM OF THE LARVA OF THE ASCIDIAN, CIONA INTESTINALIS L .1. THE EARLY LINEAGES OF THE NEURAL PLATE

The early lineages of the larval central nervous system (CNS) of the ascidian, Ciona intestinalis, have been traced using scanning electron microscopy (SEM) of embryos fixed at 12-min intervals. The CNS precursors lie superficially, exposed for a long portion (9.3 hr of 42%) of embryonic development, in the neural plate. In the 64-cell stage embryo the neural plate contains 10 cells; in all but the first vegetal division these divide with transverse cleavage planes. Synchrony is progressively lessened, but temporal sequence is always exact. Successive divisions occur initially at 30-min intervals. Our analysis confirms existing lineage descriptions for the neural plate up to the end of gastrulation and advances the lineage record through the crucial and temporally complex ninth cleavage, during which cells divide by the following rules: medial cells in each row divide first; the anterior row of vegetal daughter cells divides before their posterior siblings; the posterior row of animal daughter cells divide before their anterior siblings. All cells attain their 10th generation, but four cannot be followed by SEM. In preparation for neurulation the neural plate then comprises 76 cells, forming up to four rows each of eight vegetal hemisphere cells located on the dorsal surface of the embryo, anterior to the blastopore, and eight rows each of six animal hemisphere cells, located anterior to the rows of eight. The temporal and spatial patterns of early cleavage stages have been confirmed in vivo by observations using Nomarski optics.