What Happens in Gastrulation, and What Happens When It Goes

What happens in gastrulation, and what happens when it goes wrong?

A) Embryonic cells differentiate into the 200 types found in humans; cells don't do what they're supposed to do.
B) Genes express differently depending on which cell type they are in; cells don't do what they're supposed to do.
C) Embryonic cells move to locations where they grow into particular organs; organs don't develop correctly.
D) Embryonic cells divide and replicate; the embryo doesn't grow correctly.
E) Layers of embryonic tissue develop out of cells; organs don't develop correctly.