FIG 6. A, Normal human gastrulation. Prospective endodermal and mesodermal cells of the epiblast migrate toward the midline and ingress (arrows) through defects in the basement membrane at the primitive streak to become definitive endoderm and mesoderm.

B, Normal human gastrulation. Prospective notochordal cells located along the anterior margin of Hensen's node ingress through the primitive pit to become the notochordal process located between the ectoderm and endoderm.

C, Abnormal gastrulation resulting in a failure of midline axial integration. The primitive streak is abnormally wide; prospective notochordal cells therefore begin ingression more laterally than normal. As a result, two notochordal processes are formed. The caudal neuroepithelium, comprising two columns of tissue that flank and are separated by the primitive streak, fails to become integrated to form a single neuroepithelial sheet and instead forms two hemineural plates. The type of resultant malformation depends on the level and extent of the abnormality, and the success of subsequent reparative efforts. Reproduced from (22) with permission of S. Karger AG, Basel, Switzerland.