Epiboly is the process of

• A
  Mass migration of cells from the animal hemisphere so that upper micromeres begin to migrate over the edge of the dorsal lip and roll inside and are tucked beneath the outer layer
• B
  Overgrowth when the micromeres divide rapidly and begin to spread downwards over the megameres except at the yolk plug
• C
  Rotation of gastrula within the vitelline membrane so that the animal pole becomes anterior
• D
  Formation of a small slit-like invagination occurring on the gray crescent