Different modes of pronephric duct origin among vertebrates.

It is possible to distinguish differences in pronephric duct morphogenesis by using scanning electron microscopy to observe the results of blocking, marking and grafting experiments as well as the normal course of development. Here we compare the mode of pronephric duct development in embryos representing three orders of vertebrates: birds (class, Aves; order, Gallus); frogs (class, Amphibia; order, Anura); and salamanders (class, Amphibia; order, Urodela). The axolotl (a urodele) pronephric duct is formed by the caudal extension of a solid stream of cells segregated below somites 2 through 7. During its migration, cells are rearranged so that a short, wide rudiment is extended to form a long, thin one of similar volume. The pronephric duct rudiment of Xenopus laevis (an anuran) shows no evidence of caudal migration. Rather, pronephric duct cells are segregated out in situ by the formation of a fissure which separates them from the lateral plate mesoderm over ten somite widths. The chick pronephric duct forms a part of the intermediate mesoblast that extends by a caudal migration which does not, however, involve extensive cell rearrangements. Instead, cells near the tip of the duct rudiment in the chick proliferate, extending the duct by true growth as well as by active cell locomotion.