PHARYNGEAL POUCHES 



l6l 



the phar5ntix through narrow ducts in 10 to 12 mm. embryos (Fig. 168). When 

 the cervical sinus (p. 97) is formed, the ectoderm of the second, third, and fourth 

 branchial clefts is drawn out to produce the transient branchial and cervical ducts 

 and the cervical vesicle. These are fused at the closing plates with the entoderm 

 of the second, third, and fourth pharyngeal pouches. 



Tht first and second pouches soon differ from the others in form, and give rise 

 to an entirely different tjT)e of permanent structures. With the broadening of 



Branchial duct 2 Epithelial body of jd pouch 



Branchial cleft i 



Cervical sinus 



Pharyngeal pouch i 

 Pharyngeal pouch 2 

 Pharyngeal pouch j 



Cervical vesicle 

 Thymus anlage 

 Epithelial body of Jth pouch 



Pharyngeal pouch 4 



Trachea 



Stomach 



Dorsal pancreas 



Pharyngeal pouch 5 

 Esophagus 



Ipical bud of right lung 



Gall bladder 



Duodenum 



Fig. 168. — A reconstruction of the pharynx and fore-gut of an 11.7 mm. human embryo seen in dorsal 

 view (after Hammar). The ectodermal structures are stippled. 



the pharynx the first two pouches acquire a common opening into it. The 

 first pouch later differentiates into the tympanic cavity of the middle ear and into 

 the auditory {Eustachian) tube. By the growth and lateral expansion of the 

 pharynx, the second pouch is absorbed into the pharyngeal wall, its dorsal angle 

 alone persisting, to be later transformed into the tonsillar and supratonsillar fossce. 

 The third, fourth, axid fifth pouches give rise to a series of ductless glands,- the 

 thymus, parathyreoids, and the ultimobranchial bodies. 



