THE TONSIL 



I6 3 



12 mm. embryos (Fig. 168). When the cervical sinus (p. 91) 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. 



The fate of these entodermal pouches is varied and spectacular. 

 The first differentiates into the tympanic cavity of the middle ear and into 



Branchial duct 2 Epithelial body of jd pouch 



Branchial groove 1 



Cervical sinus 



Cervical vesicle 

 Thymus anlage 



Parathyreoid qf jib pouch. 



I'liiiryngea! pouch 1 

 Pharyngeal pouch 2 

 Pharyngeal pouch 3 



Pharyngeal pouch 4 

 Pharyngeal pouch 5 

 Esophagus 



Apical bud of right lung 



Stomach 

 Dorsal pancreas 



FIG. 168. A reconstruction of the pharynx and fore-gut of an 11.7 ram. human embryo, seen 

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



the auditory (Eustachian) tube. The second becomes the palatine tonsil 

 in part. The third, fourth, and fifth pouches give rise to a series of ductless 

 glands, the thymus, parathyreoids, and the ultimobranchial bodies. 



THE TONSIL 



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 transformed into the tonsillar and supratonsillar fosses. A mound 

 of mesodermal lymphoid tissue presses against the epithelium of the 

 tonsillar fossae in 140 (CR?) fetuses. This association constitutes the 

 palatine tonsil. Crypts arise by the hollowing of solid epithelial ingrowths. 



