1) A description of the early development of the bronchial tree in the sheep is given based on the findings of Nicolas and Dimitrova (1897) and d'Hardiviller (1897) in the sheep, and of Flint (1906) in the pig. In the summary of the findings the nomenclature used in the description of the adult bronchial tree is substituted for that used by these workers. 2) The findings of Flint (1906) on the early development of the pulmonary artery and vein in the pig are recorded, because it is thought that, with certain exceptions, these vessels will follow similar lines of development in the sheep. 3) The external appearance of the lungs is described. The term "pulmonary lobe" is defined, and based on this definition the right lung is divided into four lobes, an apical, a cardiac, an intermediate and a diaphragmatic, and the left lung is divided into two lobes, an apical -cardiac and a diaphragmatic. 4) An analysis is made of the variations which occurred in the external form of the lobes in the lungs of one hundred and fifty sheep, and possible explanations for these variations are suggested. 5) The number of subdivisions of the bronchial tree which occur between a segmental bronchus and a respiratory bronchiole are recorded in the case of three segments. The number of bronchial generations is found to be about sixteen and the number of bronchiolar generations about fifteen. 6) The broncho-pulmonary segments are described following a survey of the lungs of fifteen sheep; the right lung is found to consist of twelve segments and the left lung of eight segments. The basic international nomenclature accepted by the Thoracic Society for the human lung is adopted whenever possible. 7) Following a survey of the lungs of fifty sheep, the main branches of the bronchial tree are described, with attention to the areas which they ventilate. 8) The large branches of the pulmonary arteries are described, following a study of the lungs of fifteen sheep, with reference to the areas of the lung which they supply, and their relations to the bronchi are given. 9) The main tributaries of the pulmonary veins are described with attention to the areas which they drain. They were studied in the lungs of thirteen sheep. 10) A short discussion on the importance of a well defined pulmonary terminology is included.