i66 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



THE THYREOID GLAND 



In embryos with five to six primitive segments (1.4 mm), there 

 appears in the mid-ventral wall of the pharynx, between the first and 

 second branchial arches, a small out-pocketing, the thyreoid anlage. In 

 2.5 mm. embryos it has become a stalked vesicle (Figs. 167 B and 87). 

 Its stalk, the thyreoglossal duct, opens at the aboral border of the tuber- 

 culum impar of the tongue (Figs. 157 A) ; this spot is represented perma- 

 nently by the for amen ccecum (Fig. 180). The duct soon atrophies and the 

 bilobed gland anlage (Fig. 169) loses its lumen and breaks up into irregular, 

 solid, anastomosing plates of tissue as it migrates caudad. It takes up a 

 transverse position with a lobe on each side of the trachea and larynx 

 (Fig. 170). In embryos of 24 mm., discontinuous lumina begin to appear 

 in swollen portions of the plates; these represent the primitive thyreoid 

 follicles (Norris.) 



Anomalies Persistent portion of the thyreoglossal duct may form cysts or even fistulse. 



THE 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 



A B C 



Trachea 



Lung bud 

 Esophagus 



. E 



Apical bud 

 Primary bronchus 



Ventral bud 

 Esophagus 



Trachea 



Bronchus 



Ventral bud 



FIG. 171. Diagrams of stages in the early development of the trachea and lungs of human 

 embryos (.based on reconstructions by Bremer, Broman, Grosser, and Narath). X about 50. 

 A, 2.5 mm.; B, 4 mm.; C, stage B in side view; D, 5 mm.; E, 7 mm. 



pharyngeal pouches. This groove produces an external ridge on the 

 ventral wall of the tube, a ridge which becomes larger and rounded at its 

 caudal end (Fig. 171). The laryngo-tracheal groove and the ridge are the 



