1. The early development of the Japanese lung was studied according to the Japanese new nomenclature of the bronchial branching 1951.
2. The starting stage for the formation of the segments must be put at the Streeter's horizon XV, which stage was estimated by Streeter between 31 to 32 days old. At this developmental stage of the Japanese embryos, the right bronchus ends in three, the left one in two bulges. The right three bulges correspond to the upper, the middle, and the lower lobes, the left two to the upper and the lower lobes of the right and left lung respectively.
3. The fissures dividing each lobe are beginning to be formed at this stage, become definite during the next stage of horizon XVI.
4. At this stage of horizon XV, the lens vesicles are just closed, the early retinal pigment begins to appear in the outer layer of the retina.
5. The cerebral vesicles begin to evaginate, the nasal pits also begin to depress. This cerebral evagination and the corresponding depression of the nasal pit begins first at this stage, never in the previous stage ofhorizon XIV.
6. The so-called neural lobe in contact with the Rathke's pouch does not appear as yet. At the end of this stage, the chorda tympani communicates with the lingual nerve.
7. The axial mesenchym of the arm and leg buds becomes condensed to be called precartilaginous.
8. The arterial pattern of the aortic arches is still primitive and symmetrical.
9. The pulmonary arteries on both sides are to be followed from the sixth aortic arches respectively. Not yet any seclusion of the aortic part from the pulmonary part in the bulbus cordis is to be seen.
10. The swollen pelvic part of the metanephros can be separated from the ureter, which opens to the Wolffian duct.
11. There is no primordium of the parathyroid, the thymus and the lateral thyoid as yet in this stage of horizon XV.
12. In the next developmental stage of horizen XVI, which was estimated by Streeter 33 ± 1 days old, the formation of the segments is proceeding.
13. At the end of this stage, the right upper lobe bronchus bulges into three, the apicoventral, the subapicodorsal and ventral, the right middle lobe bronchus in two, the lateral and the medial, the right lower lobe bronchus into five, the superior the mediobasal, the ventrobasal, the laterobasal and the dorsobasal. The left upper lobe bronchus bulges into four, two apicodorsal, the subapicoventral and the lingual, the left lower lobe bronchus also into four, the superior, the ventrobasal, the laterobasal and the dorsobasal.
14. In this developmental stage of horizon XVI, the nasal pit becomes deep. At the end of this stage the mesenchymal primitive palate appears through the epithelial layer at the lower end of the pit.
15. The neural lobe in contact with the Rathke's pouch is appearing.
16. In the arm and the leg buds, there appear cartilaginous points corresponding to the humerus and femur respectively.
17. The peripheral nerves of these buds reach near the tips of them, the phrenic nerves the transverse septum.
18. The arterial pattern of the aortic arches is becoming asymmetrical in disappearing the right sixth aortiv arch beyond the branching of the right pulmonary artery.
19. There begins the seclusion of the aortic part from the pulmonary in the bulbus cordis, but there remains yet a wide common space of these parts.
20. The contact of the cranial and caudal cushions occurs also at this stage, so that the right and the left atrioventricular openings are separated.
21. The pelvic part of the metanephros becomes subdivided not only in cranial and caudal portions, but also in further subdivisions.
22. There begin to appear primordium of the parathyoid, the thymus and the lateral thyroid.

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