Fig. 7. Pronephric duct formation and posterior elongation in the urodele amphibian. (A) Dye marks (red) placed across the duct (dark gray) and adjacent tissues at the early tailbud stage show posterior extension of the duct by the late tailbud stage, rather than development in situ from underlying mesoderm. (B,C) Scanning electron micrographs (SEMs) show the posterior end of the pronephric duct at mid-tailbud (B), and the overlapping of anterior cells on more posterior ones (C, arrows). (D) SEM showing protrusions of pronephric duct cells attached to underlying intermediate mesodermal cells (arrows). Posterior is to the left in A and to the lower left in B. (A) Adapted, with permission, from Poole and Steinberg (Poole and Steinberg, 1981); (B-D) adapted, with permission, from Poole and Steinberg (Poole and Steinberg, 1977).