THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 



367 



The manner in which the bronchial rami branch is not definitely known. 

 Some maintain that the branching is dichotomous, that is, each bud gives rise 

 to two equal buds and each of these to two others, and so on. In order to as- 

 sume the adult form, however, one of the buds places itself in line with the 

 preceding bud or bronchus while the other places itself as a lateral outgrowth. 

 Others hold that the growth is monopodial, that is, that the original bud grows 

 in a more or less direct line and the others develop as lateral outgrowths. When 



Upper right lobe 



Middle right lobe 





Trachea 



Upper left lobe 



Mesoderm 

 (mesenchyme) 



Lower right lobe 



FIG. 325. Anlage of lungs of a human embryo of 4.3 mm. His. 



the evaginations that produce the bronchial rami are completed, each terminal 

 (respiratory) bronchus subdivides into three to six narrow tubules, the alveolar 

 ducts. The latter again branch into several wider compartments, the atria, 

 from which several air sacs are given off. The walls of the air sacs are evagi- 

 nated to form many closely set air cells which represent the ultimate branches 

 of the air passages of the lungs. 



Trachea 



Right bronchus 



Left bronchus 



Bronchial ramus 



Mesoderm 



(mesenchyme) 



Bronchial ramus' 



FIG. 326. Anlage of lungs of a human embryo of 8.5 mm. His. 



While there is a general tendency toward bilateral symmetry in the various 

 sets of bronchial rami, the lobes of the lungs are asymmetrical. This asym- 

 metry is indicated in the five secondary buds that arise from the two primary, 

 since three arise on the right side and only two on the left. The three on the 

 right represent the upper, middle and lower lobes of the right lung (Fig. 325). 

 The upper is known as the eparterial from the fact that its bronchus lies dorsal 



