Histology of pronephros and pronephric duct. A. Cross section of a stage 54 larva (forelimb completely developed) revealing the filtration unit and the convoluted pronephric tubule. Araldite section, 1.5 μm, stained with toluidine blue. Blood is filtered in the external glomus (gl) and the filtrate enters the coelom (co) before it is taken up into the renal tubule via ciliated nephrostomes (ne). In this late larval stage the tubule is characterized by possessing a ciliated intermediate segment (is). nc, notochord; dt, distal tubule; pt, proximal tubule. B. Longitudinal section of pronephric duct (stage 52 larva). Araldite section, 2 μm, stained with toluidine blue. The pronephric duct (pd) leaves the confines of the pronephros. co, coelom; mu, muscle; pt, proximal tubule. INSERT: The Wolffian duct (wd) at the level of the caudal part of the mesonephros in a stage 54 larva. The duct epithelium consists of two cell types: principal cells and intercalated, mitochondria-rich cells (mr). C. Transmission electron microscopy of pronephric duct shown in figure 2B. The duct is composed of a single cell type characterized by a relative smooth apical surface with few microvilli (mv) and a well developed basal labyrinth (bl) formed by the highly invaginated basal and to some extent lateral cell membranes. nu, nucleus.