. The American journal of anatomy . bsequently differentiatesinto two components, the lung-sac proper and the extrapulmonarybronchus (fig. 7^). The former expands and becomes demarkedfrom the bronchus by its increased lumen. The ental surfaceis covered by respiratory epithelium. The bronchus retains itsuniform caliber and, both in the character of its lining epitheliumand in the accessory differentiations of its enveloping meso-derm, conforms to the structure of the trachea. The smoothwalled primitive lung proper forms the direct caudal continua-tion of the extrapulmonary bronchus, as seen in
. The American journal of anatomy . bsequently differentiatesinto two components, the lung-sac proper and the extrapulmonarybronchus (fig. 7^). The former expands and becomes demarkedfrom the bronchus by its increased lumen. The ental surfaceis covered by respiratory epithelium. The bronchus retains itsuniform caliber and, both in the character of its lining epitheliumand in the accessory differentiations of its enveloping meso-derm, conforms to the structure of the trachea. The smoothwalled primitive lung proper forms the direct caudal continua-tion of the extrapulmonary bronchus, as seen in the Urodelelung (Necturus), and in the early ontogenetic stages of themammalian organ. The hilus affording entrance to the bronchus,accompanied by the pulmonary vessels, occupies its cranial entire pulmonary apparatus thus divides into a conductoryand a respiratory component. 158 GEO. S. HUNTINGTON In the subsequent stages increase in the extent of the respira-tory epitheUal area is obtained by: A. Structural modifications. Fig. 7 Schema of phyletic evolution of the vertebrate lung. Conductorypaths cross-hatched; respiratory areas stippled. of the primitive lung-sacs. B. Structural modifications of theprimitive extrapulmonary conducting canals of the trachea andbronchi. PULMONARY EVOLUTION IN MAMMALIA 159 A. Structural modifications of the primitive lung-sacs The further development of the primitive smooth walled lung-tube depends upon two subsequent changes: /. With the extension of the lung-sac its circumference de-velops points of increased epithelial proliferation. These areasof heightened mitotic activity protrude as hollow epithelial buds,whose lumina are in open communication with the central cavityof the lung-sac. The latter thus becomes studded with closelyaggregated pulmonary vesicles or crypts (fig. 7^). The lung of the Anure amphibian (Rana) furnishes a goodexample of this phyletic stage. As the surface area of the lung-sac increases the previous uni-form b
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectanatomy, bookyear1920
