THE RESPIRATORY ORGANS. 



259 



FIG. 294. 



when its expanded extremity divides into two lateral diverticula 

 of these, the right, which is the larger and longer, 

 subdivides into three branches, while the left 

 bears but two. These pouches correspond to 

 the future lobes, and thus early establish the 

 asymmetrical division of the future lungs. The 

 pharyngeal end of the pulmonary tube becomes 

 the larynx, while the remaining portions form 

 the system of air-passages, including the tra- 

 chea and the bronchial tree. In the further 

 development of the bronchial ramifications the 

 same general plan of division is repeated. The 

 already-existing tube divides dichotomously, 

 but the limbs of the forks grow unequally, since 

 the ventral bud becomes the continuation of 

 the stem, while the other becomes a lateral 

 branch. After the entire system of air-passages 

 is established the expanded ends of the terminal 

 buds produce the ultimate divisions of the pul- 

 monary structure. While the entodermal 

 diverticulum thus takes part in the formation 

 of the entire pulmonary tract, its contribution 

 is limited to the epithelial lining of the alveoli 

 and the air-tubes, while the remaining constituents of the respiratory 

 organs are derived from the mesoderm. The mesodermic tissue 



Part of sagittal section 

 of eleven- day rabbit em- 

 bryo, showing pulmonary 

 evagination: P, primitive 

 pharynx ; r, o, respiratory 

 and oesophageal tubes; b t 

 body- cavity ; m, mesoder- 

 mic tissue. 



FIG. 295. 



Portion of section of thirteen-day rabbit embryo, including developing lungs ; mesodermic pulmo- 

 nary masses, L, L', are covered with primary pleural endothelium and penetrated by bifurcations of 

 primary bronchi (/, /) ; v, blood-vessels ; o, oesophageal tube. 



surrounds the entodermic diverticula, constituting for a time a con- 

 spicuous mass, into which the epithelial tubes grow. Subsequently 

 the mesodermic area becomes so completely invaded by the rapidly- 



