Nongastrulated epiblast

The ectoderm is derived directly from which of the following?
1.Hypoblast
2.Epiblastic cells that undergo gastrulation
3.Mesoderm
4.Endoderm
5.Nongastrulated epiblast

The nongastrulating cells of the epiblast form the ectoderm (epidermis, epidermal appendages, and the nervous system). During the second week of development, the embryoblast gives rise to two primitive germ layers, the epiblast and the underlying hypoblast (answer a). At the beginning of the third week, cells from the epiblast (answer b) migrate toward the midline (primitive streak) and move inward (gastrulation). The migrating epiblast cells displace the hypoblast cells to the periphery to form the endodermal lining (answer d) of the digestive tract and form an intermediate layer of mesoderm (answer c) that will give rise to muscle, bone, and cartilaginous structures.