The significance of rudimentary nephrostomial tubules for the origin of the vertebrate gonad

Initial gonadal development was studied in 30- to 40-day-old bovine embryos . The results were interpreted in conjunction with findings on pro- and mes onephric organization in larval forms of Ichthyophis kohtaoensis (Gymnophio na, Amphibia). In bovine embryos vestigial nephrostomial tubules are the im mediate precursors of the blastemas for adrenocortical, rete, gonadal and M ullerian infundibular development. From the study of Ichthyophis it can be concluded that the vestigial nephrostomial tubules seen in the bovine embry o are pronephric and not mesonephric in nature. As a consequence, the indif ferent mammalian gonad is defined as a modified homologue of the pronephros situated in the zone of pro-/mesonephric overlapping. Such an overlapping of the two kidney generations in the fully developed state is clearly seen in Ichthyophis. Overlapping of the mesonephros with the modified pronephros (gonad) is necessary to allow intercalation of mesonephric tubules (effere nt ductules in mammals) into the male seminal excurrent duct system.