322 



THE ORGANS 



system appears as a dipping down of the entoderm of the floor of the primitive 

 pharynx (some investigators describe two original evaginations, one for each 

 lung). The tubule thus formed divides into a larger and longer right branch, 

 which subdivides into three branches corresponding to the three lobes of the 

 future right lung, (Fig. 220, b, b, b) and a smaller and shorter left branch, which 

 subdivides into two branches corresponding to the two lobes of the future left 

 lung. By repeated subdivisions of these tubules the entire bronchial system is 

 formed. Up to this point (about six months in human foetus) the development 



Fig. 220. — Scheme of Development of Lung (Right), b, b, b, the three primary bron- 

 chial buds; b' , b', b', collateral branches and secondary buds, terminating in vpp, the 

 primary vesicles; vpd, pulmonary vesicles proper or alveoli; ca, alveolar canals. The 



broken line limits the stage of the three primary lung buds; between this line 



and the line only the three primary buds, their collaterals and secondary buds; 



between the line and line , the stage of dichotomous division and of 



termination in primary vesicles. Up to this point the development is that of a com- 

 pound alveolar gland. From this line to the surface represents the final period of 

 development, which is peculiar to the lung and results in the formation of the pulmonary 

 alveoli. (Prenant.) 



is that of a compound alveolar gland, (Fig. 220). The last to develop are the 

 respiratory divisions of the bronchi with their alveolar passages and alveoli. 

 The appearance of the alveoli is wholly characteristic of lung (Fig. 220, vpd.) 

 The epithelium of the alvuoli is at first entirely of the fcEtal-cell type, the large 

 flat respiratory plates appearing only late in foetal life. Just how and when 

 the flattening of the epithelial cells takes place is not definitely known. The 

 accepted theory has been that the cells become flattened rather suddenly at birth 

 as a result of the first inspiration. Some authors describe a gradual thinning of 

 the cells from the sixth foetal month on. Bikfalir describes a gradual thinning 



