LARYNX, TRACHEA AND LUNGS 



169 



ment has dichotomous branching of the bronchi and the formation of two 

 equal buds been described. Such buds, formed dichotomously.^do not 

 remain of equal size (Flint) . 



Mediastinum 



PI euro-peritoneal membran> 



Visceral pleura^ 

 Coronary ligament 



Inferior vena cava 



Parietal pleura 



Plenral cavity 

 Pleura-peritoneal membrane 



Esophagus 

 Sinusoids of liver 



Ductus venosus 



Falciform ligamcn 



Wall of umbilical cord 



FIG. 175. Transverse section through the lungs and pleural cavities of a 10 mm. human 



embryo. X 23. 



The entodermal anlages of the lungs and trachea are developed in a 

 median mass of mesenchyme dorsal and crainial to the peritoneal cavity. 

 This tissue forms a broad mesentery termed the mediastinum (Fig. 175). 

 The right and left stem buds of 

 the. lungs grow out laterad, carry- 

 ing with them folds of the 

 mesoderm. The branching of the 

 bronchial buds takes place within 

 this tissue which is covered by the 

 mesothelial lining of the body 

 cavity. The terminal branches 

 of the bronchi are lined with ento- 

 dermal cells ; these flatten out and 



Pulmonary artery 



, ^^ Pulmonary vein 



XV? -' : '^ N , 



FIG. 176. Ventral view of the lungs of a 10.5 

 mm. embryo, showing the pulmonary arteries 

 and veins (His in McMurrich). X 27. Ep., 

 Apical bronchus; /, //, primary bronchi. 



form the respiratory epithelium of 



the adult lungs. The surrounding 



mesenchyme differentiates into the 



muscle, connective tissue, and 



cartilage plates of the lung, tracheal, and bronchial walls. Into it grow 



blood vessels and nerve fibers. When the pleural cavities are separated 



from the pericardial and peritoneal cavities, the mesothelium covering 



