Abstract: An explant assay based on embryonic tissue from laboratory animals may be used to identify substances that promote neural tube defects—teratogens to be avoided in pregnancy, and substances that inhibit neural tube defects—supplements that might be taken with benefit during pregnancy—to help reduce the incidence of birth defects. The assay screens for compounds that improve or impede mesodermal cell migration from an explant containing the primitive streak and parts of the lateral mesodermal wings that have been dissected from the embryo of a mouse or other experimental animal. Impaired migration of nascent mesodermal cells in the primitive streak is the morphogenetic basis underlying the pathogenesis of neural tube defects. We also hypothesize that maternal diabetes adversely affects mesoderm during gastrulation, before neurulation occurs.