Fig. 3. Confocal sections from embryos at late stage I (left column; approx. 16 hours AF), mid-stage II (middle column; approx. 17 hours AF) and early stage III (right column; approx. 18 hours AF). (A-C) Dorsal notochord plate. (D-F) Ventral notochord plate. Arrows indicate the lateral folds in D. In A, the secondary notochord cells (violet) are just rounding up to divide. Many primary cells have lost contact with the blastopore lip and joined the interior of the notochord plate, while others have greatly constricted apices (asterisks). (G-I) Lateral sections through the notochord plate. G and I show midline views. H shows a more lateral view to illustrate how cells at the lateral invaginating edges of the notochord plate are also elongated perpendicular to the AP axis. Note how the dorsal (basal) ends of many individual cells in I tend to extend posterior relative to the ventral (apical) ends of the same cells. The same phenomenon is apparent in F. The apices of these cells tend to become stretched along the AP axis by early stage III (not shown). Asterisks in I mark dividing cells within the neural tube. (J-L) Cross sections through the notochord plate taken near its anterior end where the lateral folds are least pronounced and the invagination progresses simultaneously across the width of the plate.
