The differentiation of the brush border which makes up the apical free surface of intestinal absorptive cells has been studied by electron microscopy. Specimens of Xenopus small intestine were fixed at various stages during metamorphosis, the time when a new intestinal epithelium forms. The interpretation of details described herein emphasizes the role of "surface-forming" vesicles. These vesicles are thought to provide membrane both for the initial expansion of the apical surface and for the later elongation of the microvilli. The latter are believed to be "molded" around filamentous cores that appear early in differentiation. The cores are attached to the apical membrane and extend vertically into the supranuclear cytoplasm. This interpretation rests chiefly on (a) the resemblance, both in morphology and in staining properties with colloidal thorium, between the membrane that limits the vesicles and that which limits the microvilli and (b) the distribution and time of appearance of the vesicles with respect to development of the microvilli. According to this view, the specific properties of surface membrane reside in preformed units that arise within the supranuclear cytoplasm. This morphogenetic process probably involves participation of the Golgi region as the site where the complex macromolecular architecture of the cell surface is assembled.