Splanchnic Mesoderm surrounding the foregut, supporting structures of the tracheobronchial tree, including muscle and connective tissues, develop from the mesoderm that surrounds the developing lung bud. the respiratory bud (lung bud) that developed at the caudal end of the laryngotracheal diverticulum (week 4) divides into 2 lateral outpouchings --> called primary bronchial buds.