Zebrafish Notogenesis and Early Neurulation. Mediolateral intercalation produces convergent-extension of the notochordal rudiment. Cells within the center of the notochord elongate and are incorporated into the notochordal-somitic boundary. The medial thickening of the neural plate becomes visible along the embryonic midline. As the neural plate converges toward the embryonic midline, the notochord becomes depressed ventrally. Oriented neuroepithelial cell divisions first take place in the ventral medial thickening (i.e. presumptive floorplate) near the neural midline after mediolateral intercalation is well-advanced in the notochordal rudiment. The otic placode comes visible as the embryo undergoes a torsional rotation.