Practice question

Epiboly describes overgrowth and spreading of ectodermal epithelial sheets to enclose underlying embryonic structures such as mesoderm, endoderm or yolk mass. During gastrulation epibolic movement thins surface layer via radial intercalation while expanding its area, sealing embryonic blastopore or wrapping yolk syncytial layer. This enclosure creates continuous outer epithelium that later becomes epidermis and protects internalizing progenitors, ensuring proper body plan. It does not directly form neural tube, which relies on neurulation folding, but provides substrate and positioning. Failure results in exogastrulation and incomplete envelopment of vegetal hemisphere.