Fig. 2. The relationship of the embryonic dorsal axis to the definitive dorsal axis. (A) The cell movements of gastrulation obliterate the blastocoel, enclose a new cavity (the archenteron) and move cells into new positions. A patch of cells (red) in the region of the prechordal mesoderm lie above the dorsal lip of the blastopore at stage 10.25 (early gastrula stage). These cells move animalwards during gastrulation, away from the site of the blastopore lip, along the blastocoel roof to become part of the anterior (head) mesoderm by the end of gastrulation (stage 12). Meanwhile, the convergence extension of the chordamesoderm encloses the vegetal mass, and by the end of gastrulation, the definitive dorsoventral axis can be defined by a line at right-angles to the definitive anteroposterior axis. The obliterating blastocoel (black) lies on the ventral surface and the closing blastopore marks the definitive posterior end. (B) A comparison of the embryonic axes, at the early gastrula stage (stage 10.25), and the definitive axes, at the end of gastrulation (stage 12).