Fig. 6. Cell movements at the dorsal lip of the blastopore during early gastrulation. During the first 2 hours of gastrulation, the vegetal mass undergoes an active inward surging (black arrow) in the animal direction, such that the animal part of the vegetal mass expands, increasing the area of the blastocoel floor, while the vegetal part contracts. This movement drives the first phase of involution of the prechordal mesoderm (red). After involution, prechordal mesoderm cells become actively migratory and migrate animally. Their movement is stopped by mid-gastrula stage by their firm adhesion to the substrate, the proneural animal cap cells. The chordamesoderm (purple) then undergoes convergence extension movements to cover the vegetal mass and close the blastopore. The bottle cells (black) indicate the site of dorsal lip formation.