Epiboly is the process of

• A

  Rotation of gastrula within vitelline membrane so that animal pole becomes anterior

• B

  Overgrowth of micromeres which divide rapidly and spread downward over megameres except at yolk plug

• C

  Mass migration of cells from animal hemisphere so that upper micromeres migrate over edge of dorsal lip, roll inside and tucked beneath outer layer

• D

  Formation of small slit like invagination upon grey crescent