176 



THE ECTODERMAL CANAL AND ITS DERIVATIVES 



Mzdiasti 



the formation of two equal buds been described. Such buds formed dichoto- 

 mously do not remain of equal size (Flint, Amer. Jour. Anat., vol. 6, 1 906-1907). 



The entodermal anlages 

 of the lungs and trachea are 

 developed in a median mass 

 of mesenchyma dorsal and 

 cranial to the peritoneal 

 cavity. This tissue forms a 

 broad mesentery termed the 

 mediastinum (Fig. 169). The 

 right and left stem buds of 

 the lungs grow out laterad, 

 carrying with them folds of 

 the mesoderm. The branch- 

 ing of the bronchial buds 

 takes place within this tissue 

 which is covered by the 

 mesothelium which lines the 

 body cavity. The terminal 

 branches of the bronchi are 

 lined with entodermal cells which flatten out and form the respiratory epithelium 

 of the adult lungs. The surrounding mesenchyma differentiates into the muscle, 



Inf. vena 



cava 



Fatcifg, 



Wall of umbilical cord 



Fig. 169. — Transverse section through the lungs and pleural 

 cavities of a 10 mm. human embryo. X 23. 



Fig. 170. — The lungs of a 10.5 mm. embryo showing the pulmonary arteries and veins (His from 

 McMurrich's "Human Body"). Ap., pulmonary artery; Ep, apical bronchus; V p, pulmonary vein; 

 /, //, primary bronchi. 



connective tissue and cartilage plates of the lungs, trachea and bronchial walls. 

 Into it grow blood-vessels and nerve fibers. When the pleural cavities are 



