THE EESPIRATOKY SYSTEM. 



251 



In the 4th week the pulmonary pocket produces a larger 

 right and a smaller left process, the right and left lung buds. 

 The median part, developed from the hinder end of the groove, 

 grows out from the pharyngeal floor and forms the trachea. 

 The groove in front forms the larynx. The right bud forms 

 the right lung and bronchus ; the left, the left lung and bronchus. 

 The hypoblast becomes the epithelial lining of the respiratory 

 tract ; the surrounding mesoblast forms the vessels, connective- 

 tissue covering and coats of the respiratory tubes and lungs. 

 As the lung buds develop the stomach is forced backwards ; 

 the oesophagus becomes elongated. The tracheal part of the 

 bud becomes separated from the oesophagus, but both retain the 

 same nerve supply — the recurrent branch of the vagus — which 

 is the nerve of the 5 th arch. 



right dorsal aorta 



fore-gut 

 right lung bud 

 dorsal mesocard. 



passage between 

 pericard. and pleura 



left dorsal aorta 

 dorsal mesent. 



card, uein of left side 



-f-pleuro-perit. eommun 

 pleura 



uen. mesocard. 

 left d. of Cuuier 

 pericard. 

 heart 



pericardium 



Fio. 205.— A section of a human embryo to show the Relationships oJ the Pulmonary 

 Buds at the 4th week, looking backwards. (After Kollmann.) 



In Fig. 205 the relationship of the lung buds is shown to 

 surrounding structures during the 4 th week. The following 

 points should be noted : 



(1) As the lung bud grows out it pushes its way into the 

 isthmus of the coelom — the narrow neck of communication 

 between the pericardium and peritoneum (Fig. 201). This part 



