1. The pronephros opposite any given somite in the chick is probably determined just before the early neural plate stage at that level, and thereafter develops independently of the somite or lateral plate mesoderm.
2. The Wolffian duct, which is first formed by the union of the pronephric tubules, grows backwards by the multiplication of its own cells. If this elongation is checked by a transverse cut made posterior to the end of the duct, the duct does not appear behind the cut.
3. In the absence of the Wolffian duct, the mesonephros is not properly developed, appearing only as very small patches of tissue with no lumen. The Wolffian duct may therefore be said to induce the mesonephros, which, however, has a feeble power of self-differentiation as a double assurance.
4. This provides a confirmation of previous suggestions that an organ which is vestigial in the sense that it has lost its physiological function may be retained in ontogeny because it still fulfils the morphogenetic function of providing a stimulus essential for the development of other, physiologically more important, structures.