LARYNX, TRACHEA AND LUNGS 



173 



as it migrates caudad. It takes up a transverse position with a lobe on each side 

 of the trachea and larynx (Fig. 164). In embryos of 50 mm., lumina appear in 

 the more peripheral cords which break up into hollow or solid groups of cells, the 

 primitive thyreoid follicles (Grosser). 



LARYNX, TRACHEA AND LUNGS 

 In embryos of 23 segments, the anlage of the respiratory organs appears as a 

 groove in the floor of the entodermal tube just caudal to the pharyngeal pouches. 

 This groove produces an external ridge on the ventral wall of the tube, a ridge 



Trachea. 



RespiraTory 

 anlage 



Esophagus 



D 



Trachea. 



Apical bud 



Primary 

 bronchus 



Esophagus 



Trachea. 



Bronchus 

 Ventral bud 



Fig. 165. — Diagrams of stages in the early development of the trachea and lungs of human embryos 

 (based on reconstructions by Bremer, Broman, Grosser, and Naroth). A, 2.5 mm.; B, 4 mm.; C, B in 

 side view; D, 5 mm.; E, 7 mm. 



which becomes larger and rounded at its caudal end (Fig. 165). The laryngo- 

 tracheal groove and the ridge are the anlages of the larynx and trachea. The 

 rounded end of the ridge is the unpaired anlage of the lungs. 



■ Externally two lateral longitudinal grooves mark off the dorsal esophagus 

 from the ventral respiratory anlages. The lung anlage rapidly increases in size 

 and becomes bilobed in embryos of 4 to 5 mm. The lateral furrows become deeper 

 caudad and a septum is formed which grows cephalad, separating first the lung 

 anlages and then the tracheal tube from the esophagus. At the same time the 

 laryngeal portion of the groove and ridge is developed cranially until it lies be- 

 tween the third and fourth branchial arches (Fig. 82). At 5 mm. the respiratory 



