The development of the dog lung during the foetal life (10-120 mm CVR length) was studied. At 10-15 mm CVR length, the developing lung appeared in the caudal part of the roof of the thoracic cavity. It was represented by tubular structures lined with pseudostratified epithelium embedded in mesenchymal connective tissue mass which was surrrounded by cuboidal or flattened mesothelium. On reaching 20-30 mm CVR length, the lung extended cranially to reach the thoracic inlet and ventrally to enclose the heart and its surrounding pericardium. The epithelial portion of the primordium of the lung appeared as a primitive system of branched epithelial tubules. These tubules became more branched with the advancement of age and differentiated into light and dark tubules (the prospective conducting and respiratory systems, respectively), at 50 60 mm CVR length. The adult form of the lung concerning the lobation and fissures were observed at 70 mm CVR length. Around 110-120 mm CVR length, the dark tubules were increased in their number coinciding with the decreased number of the light tubules The dark tubules (acinar tubules) had a glandular appearence as they closely resembled glandular end-pieces. The light tubules showed coarse and fine PAS positive granules while the dark ones showed few, fine granules. The epithelium of the light tubules (future bronchial tubules) demonstrated large number of disintegrated cells with pyknotic nuclei. In this period of intrauterine life, the pulmonary blood vessels showed a peculiar stage of development.