CHAPTER XIII. 

 THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 



The anlage of the respiratory system appears in human embryos of about 

 3.2 mm. A hollow, linear evagination— the lung groove— develops on the 

 ventral side of the oesophageal portion of the primitive gut, extending caudally 

 a short distance from the region of the fourth inner branchial groove. It was 

 once thought that the evagination developed along practically the entire length 

 of the oesophagus anlage, but more recent researches seem to prove that it is 

 confined to the cephalic end. The lung groove soon becomes separated from 



Pharynx 



Branchial arches 

 (pharynx) 



Hypophysis 



Yolk sac 



Belly stalk 

 Caudal gut 



Mesonephros 



Allantoic duct 



Hind-gut 



Kidney bud ' 

 Fig. 320. — Sagittal section of reconstruction of a human embryo of 5 mm. His, Kollmann. 



the gut by a constriction which appears at the caudal end and gradually pro- 

 gresses forward. Thus there is formed a tube which lies ventral to the gut and 

 which opens upon the floor of the latter at the boundary line between the 

 oesophagus and pharynx (Figs. 320 and 284). 



From this simple tube the entire respiratory system develops. The 

 cephalic end gives rise to the larynx, the opening into the gut being the aditus 

 laryngis. The middle portion gives rise to the trachea. Two outgrowths from 

 the caudal end of the tube, which appear about the time of separation from the 



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