Experimental research on developmental mechanisms has shown that the processes involved in the differentiation of organs are the results of long sequences of interactions among cells and cell groups (Weiss and Taylor,'60). The present research was aimed at deciding whether in a single embryonic system (pulmonary) different areas of the apparently homogeneous mesenchyme have diverse inductive actions upon the epithelial tissue.

This question arose from an incidental observation on the capacity of mesenchyme to recombine after tryptic separation and then to resume morphogenesis in vitro (Alescio and Cassini,'61). Orally to its bifurcation, the tracheal epithelium occasionally gave origin to supernumerary buds; this anomaly never was seen to OC-cur, however, when the lung as a whole was explanted (Alescio,'60). A similar effect had been found by Grobstein ('53a). He obtained budding from the submandibular duct which was explanted in vitro under the condition of contact with capsular mesenchyme. It therefore was believed worthwhile to investigate whether also in the lung our anomalous budding of the tracheal epithelium had been induced by a casual contact with pulmonary (bronchial) mesenchyme. We found, as we shall report, that this effect was reproducible.