Phases in the lung development
The embryonic period

At S12, the ventrally-located lung diverticulum divides into a right and a left lung bud. At S13-14, the two buds develop into the main bronchi and the common stem becomes the trachea. The left lung bud is slightly pushed away by the heart loop and so grows in a more lateral direction, whilst the right lung bud is directed caudally. In this way, both the asymmetry of the lungs and the different angles of the main bronchi growing away from the trachea are defined. At S14, the two lung buds elongate to become lung sacs. At S16, this is where the primordia of lobar buds appear (three on the right, two on the left). At S17, segmental buds, and thus also segmental bronchi, arise. The embryonic period, which extends from weeks 4 to 7, is followed by four phases of lung development in the fetal period. These phases partly overlap.