Early Gastrulation
The marginal zone cells involute when they reach the dorsal lip of the blastopore, while the animal cells undergo epiboly and converge at the blastopore. The archenteron forms and starts to displace the blastocoel. Ventral and lateral blastopore lips form and endodermal and mesodermal precursor cells migrate from the lateral and ventral lips of the blastopore into the embryo. The mesodermal cells converge and extend along the antero-posterior axis.