Changes

*When the embryo is in it's blastula stage it has two cell layers; the epiblast and hypoblast.

*When the embryo is in it's blastula stage it has two cell layers; the epiblast and hypoblast.

**The epiblast is columnar whilst the hypoblast is cubiodal - each is a single cell thick.  

**The epiblast is columnar whilst the hypoblast is cuboidal - each is a single cell thick.  

*'''The epiblast gives rise to all three cell layers; the hypoblast makes no contribution'''.  

*'''The epiblast gives rise to all three cell layers; the hypoblast makes no contribution'''.  

*The endoderm and mesoderm develop by proliferation and migration of cells of the epiblast.  

*The endoderm and mesoderm develop by proliferation and migration of cells of the epiblast.