A graphical view of the four stages in spinal cord development.
A. Neural plate stage: the neural ectoderm invaginates along its central axis.
B. Neural fold stage: neural folds develop on either side of the neural ectoderm borders. At this stage, the floor plate cells and the paraxial mesoderm (somitic mesoderm) begin to develop.
C. Neural tube stage: the neural tube is formed and roof plate cells begin to differentiate at the neural tube's dorsal midline (orange).
D. Spinal cord stage: Motor neurons (M) and ventral interneurons (V) develop within the ventral half of the spinal cord.