The dorsal and ventral hemispheres at this stage are mirror images of one another. The yellow pigment lies in four posterior cells. They give rise to mesoderm and mesenchyme. Six yolk-filled cells at the vegetal pole, anterior to the yellow mesoderm cells, give rise to endoderm. All the other cells are ectodermal.
Gastrulation is by epiboly.
They overgrow it, finally engulfing the yellow cells which are then seen only dimly through the translucent ectoderm. Studies in tunicate development. I. General physiology of development of simple ascidians. Culture methods for ascidians. The early development of ascidian eggs.