Fig. 7. A model of anterior fate determination in the ascidian ectoderm at the eight-cell, 64-cell and neural plate stages. The anterior ectodermal lineage (red) gives rise to the trunk epidermis (not coloured), the palps (striped motif) and the sensory vesicle (red), while the posterior ectodermal lineage (green) gives rise to the tail epidermis (not coloured) and the dorsal row of the nerve cord (green) at the larval stage. At the eight-cell stage, Ci-FoxA-a expression is activated in the anterior animal cells. Presence of this factor leads to the activation of anterior ectodermal genes at the 64-cell and neural plate stages, and to the repression of posterior ectodermal genes. Vegetal expression of Ci-FoxA-a is not represented in this model.