Though many experiments concerning the development of the avian lung anlage have been performed, our knowlegde is still inadequate and defective with regard to the state of the development of air-sacs, parabronchi and recurrent bronchi. The author investigated to the development of the lung anlage of Uroloncha domestica Flower, comparing his results with those of Rösler, Shikinami, Murayama and Yakushiji, and in connection with this, further observed the state of the dovelopment of air-sacs, parabronchi and bronchial circuits. Most of the embryos were fixed in Zenker's fluid, and stained with borax-carmine. After subsequent treatment in the usual manner, these specimens were embedded in paraffin and trimmed and cut in transverse serial sections with a thickness of 10 microns. A number of models were prepared according to Born-Peter's method of wax-plate reconstruction. From the results obtained the following may be concluded. 1) The lung anlage of Uroloncha domestica Flower develops in pairs as in the case of the mammals and other birds. 2) In an embryo of 28 somites and 4.0mm in length, the first appearance of the lung anlage comes in as an enlargement of each side of the foregut caudal to the third gill-pouch. 3) In an embryo of 3.5mm length and of 31 somites, the lung anlage appears as a hernious enlargement on each side of the foregut. The distal end of the anlage foreshadows the bronchus anlage. 4) In an embryo of 4.5mm in length, the bronchus anlage is clearly distinguished and its distal portion is divided into the right and left mesobronchi. Each of the mesobronchi, markedly extending in the caudal direction, shows in the central portion of the dorsal surface the first division of the bronchus, i.e. Ent. 1., as a papillar bud-like enlargement. 5) In an embryo of 5.0-6.5mm length, each mesobronchus, divided from the bronchus, shows irregular spindle-shaped enlargement which forms the embryonic vestibulum. In an embryo of 7.0mm length, the embryonic vestibulum, surrounded by the mesobronchus, does not show any clear outline. 6) In an embryo of 5.0mm in length, the mesobronchus shows not only the Ent. 1. and 2., and at the caudo-medial portion Ent. 3. and 4., but also Ect. 1. and 2., which developed from the dorsal surface of the embryonic vestibulum. The four entobronchi enumerated in Uroloncha domestica Flower and these are in front of the embryonic vestibulum. 7) In an embryo of 5.5mm length, not only do each of the Ent. 1., 2., 3. and 4. show enlargement and proliferation, but the Ect. 1., 2. and 3. develop from the dorsal surface of the embryonic vestibulum, and the Lat. 2. is foreshadowed at the lateral wall of the embryonic vestibulum. 8) In an embryo of 6.0mm length, three branches from Ent. 1., extending towards cranial, transverse and medial, two branches from Ent. 2. towards ventro-lateral and dorso-lateral, two branches from Ent. 3. towards cranial and ventral, develop in connection with the enlargement of Ent. 4. The outlines of Ect. 1., 2., 3., 4., and Lat. 1., 2., 3. are clearly seem. 9) In an embryo of 6.5mm length, the first appearance of the air-sacs and parabronchi, which are proper to the bird's embryo, is observed. It arises from the Ent. 1., cranial the cervical air-sac, transversal the lateral moiety of the interclavicular airsac: from the Ent. 3., cranial the medial moiety of the interclavicular air-sac, ventral the anterior intermediate air-sac: from the Lat. 3., the posterior intermediate air-sac: from the Ent. 1. and 2., respectively 3. and 2. parabronchi: from the Ent. 3., each two parabronchi of the ventral and dorsal walls, and from Ent. 4. none. Further the ectobronchi develops 4 on the right side and 5 on the left. Ect. 1. divides in cranial and dorsal branches, from each of which two parabronchi develop. 3 parabronchi rise from Ect. 3.,4 . from Ect. 3.,a nd 2., from Ect. 4. At this time, Lat. 1., 2.,a nd 3. begin to appear and one parabronchus develops from Lat . 2. 10) In an embryo of 7.0mm length, each bronchial tree shows not only more enlargement and ramification, but each branch arising from the bronchial tree also shows marked development and grape-like appearance where they come in contact with each other. Ent. 1., and 2. from the ventral portion of the upper region of the lung. The tree-like parabronchi arising from Ent. 1., and 2., come in contact at the lateral surface of the upper region of the lung with the parabronchi from Ect. 1., 2. and Lat. 1., 2., which extend in a cranial direction. Ent. 3., and 4. from the ventral portion of the caudal lung region, and the parabronchi arising from these come into contact at the lateral and medial sides of the dorsal surface of the lung with those from the Ect. 2., 3., and 4. In this way, the parabronchi which come in contact with each other, combine together and from the bronchial circuits. And the anterior-intermediate-air-sac of the Ent. 3., as mentioned in the embryo of 6.5mm length, extending towards ventral, forms a rod-like prominence and develops the medial moiety that extends from cranial to ventral direction. At this stage, the first appearance of the recurrent bronchi is observed at the cranial extremity of the posterior-intermediate-air-sac, rising from Lat. 3.