Expression of GAD during amphioxus development. Whole mounts with anterior to the left. The cross-sections in F-H are viewed from the caudal end of the animal. Whole mount and section scale bars are 50 μm and 25 μm, respectively. A: Side view of a 16-hr neurula showing three spots of expression in the neural tube. B Magnification of a 18-hr neurula showing three pairs of GAD-positive neurons. C Anterior region of a 24-hr larva. D Dorsal view of the embryo in C showing conspicuous expression in clustered neurons (arrow) at the posterior end of the cerebral vesicle. At more posterior levels of the neural tube, there are two more clusters of labeled neurons (black and red arrowheads). E: Lateral view of a 36-hour larva. F-H: Cross-sections of the larva shown in E through levels f, g and h, respectively. I: Side view of 48-hour larva. The inset shows GAD-positive sensory cells around the oral cavity (arrowheads). N, notochord.