A SEM study of palatal shelves from therapeutically aborted human embryos revealed that specific alterations occur in the epithelium of the medial edge prior to contact between the shelves. These alterations include elongation and flattening of surface cells, apparent cell death and desquamation, and overall thickening of the medial edge epithelium. The progressive spread and increasing extent of these alterations precedes and mimics the pattern of shelf contact and fusion. TEM study of sections cut from these previously scanned specimens is currently under way to correlate the surface topograpy with underlying ultrastructure. SEM examination of palatal shelves from CD-1 mice and golden hamsters reveal similar specific prefusion alterations in these species. SEM examination of isolated palatal processes from mouse embyros cultured in vitro on various substrates is also in progress. Prefusion surface alterations were detected by SEM along the neural folds of CD-1 mouse and golden hamster embryos during closure of the neural tube. The development of these surface alterations has been studied from initial appearance of the neural folds to closure of the posterior neurospace. Characterization of the adenylate cyclase enzyme during prenatal development of the secondary palate in the golden hamster has been initiated. Basal enzyme activity and phosphodiesterase activity have been determined, and the sensitivity of adenylate cyclase to stimulation by epinephrine, norepinephrine, histamine, serotonin, thyrotropin, growth hormone, thyroxine, glucagon, isoproterenol, dopamine, parathyroiid hormone and calcitonin has been examined. BIBLIOGRAPHIC REFERENCES: Waterman, R. E., G. C. Palmer, S. J. Palmer and S. M. Palmer 1976 Catecholamine-sensitive adenylate cyclase in the developing golden hamster palate. Anat. Rec., in press. Waterman, R.E. 1976. Topographical changes along the neural fold associated with neurulation of the hamster and mouse. Anat. Rec., in press.