Fig. 7. Excision of ecto-mesodermal tissue lateral to the midplane, avoiding the archenteron (EM cut). (A,B) A schematic drawing (A) of ablations of ecto-mesodermal tissue lateral to the archenteron (right EM cut) and a bright field image (B) of an embryo subjected to the right EM cut. The embryos are viewed from the aboral side, with the animal side at the top. (A) The mid-sagittal plane of the embryo is represented by a broken line, and the cut plane by a broad line. (B) An S. mirabilis embryo subjected to the right EM cut. The right animal region lateral to the archenteron was removed from the embryo. The epithelium of the embryo has adhered at the cut plane so that the cut end is closed. The archenteron was not severely damaged and is almost intact. (C) An oral side view of a two-armed pluteus S. mirabilis larva subjected to the right EM cut. The animal side is towards the top, and thus the left and the right sides of the larva are on the right and the left, respectively, of the viewer. The morphology of the coelom of the right coelomic sac (arrow with r) on the cut side appears normal, the same as the left coelomic sac (arrow with l). Scale bars: 50 µm in B and 100 µm in C.