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 



Pleura-peritoneal membran 

 Visceral pleura 

 Coronary ligament 



Inferior vena cava 



Parietal pleura 



Pleural cavity 

 ^-Pleura-peritoneal membrane 



Falciform ligament 



Sinusoids of liver 

 Duclus venosus 



Wall of umbilical cord 



Pulmonary artery 



Pulmonary vein 



PIG. 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 

 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 



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; I, II, primary bronchi. 



