Fig. 5. Developmental patterns of lamprey branchial motoneurons and the reticulospinal tract. (A-C) Confocal microphotographs of a stage 26 lamprey larva. Lateral (A,C) and dorsal (B) views. In the hindbrain, the rhodamine-labeled reticulospinal tract (purple) runs below the fluorescein-labeled trigeminal (Vm) and facial (VII) motor nuclei (green). Note that the Mauthner (Mth) neuron is located between Vm and VII. (D) Stage 30 embryo. The Mth neuron is at the posterior margin of Vm (purple) as in stage 26. (E) A stage 28 larva in which facial (VII), glossopharyngeal (IX) and vagus (X) nerves have been labeled. Branchial motor nuclei (green) are arranged along the anteroposterior axis, as seen in the adult. Note that the Mth neuron (purple) appears in the rostral hindbrain and adjacent to the VIIth nucleus as in the stage 26 larva.