The embryology of spinal dysraphism.

The theories of abnormal embryogenesis suggested as explanations for the various forms of SBA and occult spinal dysraphism appear in some instances too all encompassing, as in Gardner's hydrodynamic theory or Padget's neuroschisis theory, or too limited, as in the theory of arrested development proposed by Daniel and Strich for the ACM, or just too speculative in many cases. The observation of a completed defect does not allow one to venture backwards in development to a specific time and cause with any accuracy. Perhaps several different causes striking different tissues at different times can set up a series of aberrations that lead to morphologically similar mature anomalies. The ability of developmental processes to heal themselves, as shown experimentally, may obscure the true mechanism and timing of occurrence although the final morphological expression may be dramatic. Since the study of human embryogenesis in the experimental laboratory is ethically unacceptable although technically feasible, the elucidation of the mechanisms of these neural defects will be long in coming.