Which Gastrulation Event in the Sea Urchin Occurs Later Than

Which gastrulation event in the sea urchin occurs later than the others?

A) Flattening of the vegetal hemisphere
B) Formation of primary mesenchyme when some cells at the vegetal pole break away and move into the blastocoel
C) Extension of filopodia from secondary mesenchyme to attach to the ectoderm
D) Cuboidal cells becoming wedge-shaped at the vegetal pole
E) Cell invagination to form the archenteron