Chick embryos were staged according to the method of Hamburger and Hamilton [1951] and fixed. Cross sections through the cephalic fourth of the mesonephric ridges were examined by scanning electron microscopy. The steps in glomerular differentiation could be observed with ease. The first foot processes to appear in podocytes arose directly from the basal surface of the cell body. In a second step, lateral branches appeared and gave off secondary or even tertiary branches that interdigitated with those from neighbouring podocytes, following a pattern that was very similar to the one previously described by other authors in metanephric nephrons. Endothelial pores appeared in the glomerular capillaries at very early stages of the glomerular differentiation. The differentiation of the epithelium of proximal tubules was characterized by the growth of apical microvilli and of finger-like evaginations from the lateral membranes. At stages 20 and 21, the most differentiated glomeruli had only basal foot processes; only after stage 25 did the first generation nephrons reach full maturity. Because during this period the mesonephros is known to produce urine, our results indicate that nephrons start to function before they have completed their differentiation.