Fig. 4

Differential interference contrast images and schematic cell position representations at comparable late gastrula stages in an ascidian embryo (upper part) and in a larvacean embryo (lower part). Axis orientation labels next to images indicate orientations: a anterior, a _n animal, l left, p posterior, r right, v vegetal. Absolute age is indicated by the blue dotted lines in Fig. 1; tissue fates are color coded as in Fig. 1. Note the similarity in the relative positions of the different tissues, despite the considerably lower cell number in the larvacean. Note also that prospective nervous system cells (yellow) are arranged as a neural plate at the anterior border of the blastopore in the ascidian whereas the corresponding cells have already ingressed (neurulated) in the larvacean. (For detailed spatial and temporal resolution of neural fate restrictions in the two species see Additional files 7, 8, 9 and 10). bp, blastopore