Summary The observations presented above support the previously advanced concept of the embryogenesis of human and experimental cranioschisis(3-7). They also support the idea that any mesodermal alteration causing its derangement and collapse before the complete closure of the neural tube will necessarily be followed by failure of closure of the neuro-ectoderm, regardless of the nature of the teratogenic agent used. This concept supports the following facts: the closure of the cephalic neuro-ectoderm is a critical and significant period of embryonic development; the cephalic mesoderm plays an important role in its final closure; and cranioschisis (anencephaly), the most commonly induced experimental malformation, is the direct consequence of the failure of the cephalic mesoderm to fulfill its function of enclosing and lodging the cephalic neuro-ectoderm.