Chevalier Jackson lecture. Respiratory and alimentary relations in staged human embryos. New embryological data and congenital anomalies.

The early development of the digestive and respiratory systems is summarized in relation to embryonic staging. It has frequently been emphasized that the digestive and respiratory tubes do not arise from a common chamber, that they pursue separate courses as soon as the lung bud appears, that a mesenchymal septum comes to intervene between them, and that the two tubes rapidly acquire independent outer coats. Some commonly held views such as the supposed caudorostral separation of the trachea from the esophagus, have been shown to be incorrect. These ideas often arose from the use of unstable landmarks during development. It is pointed out that, for 3 weeks after its appearance, the tracheoesophageal separation point remains at a constant level, whereas the tracheal bifurcation descends. The application of valid embryological data to the interpretation of several congenital anomalies has been shown to be of value. Although the modes of origin in some instances are still obscure, considerable advance has been made in understanding the timing of the relevant events.