Skin barrier function is conferred by the outer layer of epidermis, the stratum corneum, and is essential for terrestrial life. Quantitative trans-epidermal water loss assays show that barrier forms late in embryogenesis, permitting the foetus to survive a terrestrial environment at birth. Using qualitative in situ assays for skin permeability, we show that barrier forms in a patterned manner late in mouse gestation. Barrier forms at specific epidermal sites, then spreads around the embryo as a moving front. The moving front of permeability change is accompanied by multiple changes in the outer, stratum corneum-precursor cells. We use the permeability assays to show that final stages of cornified envelope assembly are coordinated with initial stages of barrier formation. Hence the whole-mount permeability assays record developmental acquisition of a known, essential component of the adult barrier. We demonstrate the authenticity of the whole-mount assays after maternal glucocorticoid therapy (known to accelerate barrier formation) and in additional species including the rat where barrier formation is well characterized by TEWL assay (Aszterbaum, M., Menon, G. K., Feingold, K. R. and Williams, M. L. Pediatr. Res. 31, 308-317). The demonstration of patterned barrier formation in other species suggests patterned change as the universal mode of embryonic barrier acquisition. These results highlight the importance of patterning as a mode of epidermal maturation during development.