In the lower part of the pharynx arises ventrally from the midline towards the ventral direction a groove, the laryngotracheal sulcus (stage 10, ca. 28 days). The caudal end develops to become the lung buds that, parallel to the esophagus, grow into the mesoderm. This part becomes the trachea.
Through a series of interactions with the surrounding tissue these lung buds begin to divide and engender the right and left main bronchi (stage 13-14, ca. 32-33 days). Through further subdivisions the whole left and right bronchial trees of the lungs arise step by step. (see respiration tract module)