The Morphogenetic Function of a Vestigial Organ in the Chick

THE pronephros of the chick is usually considered to be a vestigial organ. It only appears for a short period during early development and it is never functional as an excretory organ. From it, however, the primordium of the Wolffian duct is formed, by the confluence of the segmented pronephric tubules to a continuous strand of cells. This strand lengthens and extends into the posterior part of the body, although the pronephros itself does not develop behind about the sixteenth somite. At a later stage, the mesonephros, which is the functional excretory kidney during most of embryonic life, appears in close connexion with the Wolffian duct in the region from about the fourteenth to the thirtieth somite (Lillie, 1919). The purpose of the experiments recorded in this paper was to examine the morphogenetic mechanisms involved in this sequence of events. All the experiments on chicks, except a few which are specially described, were performed on embryos cultivated in vitro in the usual way. A few experiments have been made on embryos of Ranafusca and are described later.