Fig. 2. LR placement of the adult rudiment in left and right half of S. mirabilis larvae dissected in the early or mid-late gastrula stage. The LR axis of a larva coincides with that of a gastrula embryo. The axial relationship of the animal-vegetal and the oral-aboral embryonic axes is as follows: the larval oral (anterior) side corresponds to the embryonic oral side, while the larval-ventral and the larval-dorsal sides correspond to the vegetalmost and the animal-side portions, respectively, of the embryonic aboral side. The larvae are viewed from the dorsal side, with the oral side at the top. The left and right sides of the larvae correspond to those of the viewer. (A) A larva corresponding to the six-armed pluteus derived from the right half dissected in the early gastrula stage and developing an adult rudiment on the left side (arrow). (B) A larva corresponding to the six-armed pluteus derived from the left half dissected in the early gastrula stage and containing a developing hydrocoel (white arrow) on the right side. Evidence of the vestibule (black arrows) is also visible on the right side of the larva. (C) A larva corresponding to the six-armed pluteus derived from the left half dissected in the early gastrula stage, with a vestibule (black arrow) and a hydrocoel (white arrows) developing on the left side. (D) A larva corresponding to the six-armed pluteus derived from the left half dissected in the mid-late gastrula stage and developing an adult rudiment on the right side (large arrow). The left anterolateral arm (small arrow with c) and skeletal rod of the right anterolateral arm (small arrow with d) and the left (small arrow with a) and right (small arrow with e) postdorsal arms are growing in the half larva. No postoral arm is growing on the cut side of the larva, but the left postoral arm (small arrow with b) can be seen to be growing in the larva. Scale bars: 100 µm.