Prior to implantation early in the 2nd week the inner cell mass converts to an epiblast (primitive ectoderm) and a hypoblast (primitive endoderm). Beginning at the third week, gastrulation begins and mesoderm appears (also originating from epiblast). Gastrulation begins with the formation of the  primitive streak at the posterior (caudal) end of the embryo; the streak is a linear thickening on the dorsal surface of the epiblast in which cells of the epiblast form endoderm and mesoderm. A collection of cells at the end of the primitive streak is the primitive node (Hensen's node) - epiblastic cells migrate anteriorly through the node to become the longitudinally running cellular rod called the  notochord.

Under the influence of the underlying is seen lying notochord, which develops from the axial mesoderm, the dorsal ectodermal surface of the early embryo thickens and elongates to form neural plate. Subsequent changes convert the plate into a neural tube which will give rise to the CNS.
 

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