170 



THE ENTODERMAL CANAL AND ITS DERIVATIVES 



pharyngeal pouches soon connect with the pharyngeal cavity through wide open- 

 ings. The third and fourth pouches grow laterad and their diverticula com- 

 municate with the pharynx through narrow ducts in 10 to 12 mm. embryos 

 (Fig. 162). When the cervical sinus is formed the ectoderm of the second, third 

 and fourth branchial clefts is drawn out to produce branchial and cervical ducts and 

 the branchial vesicle. These are fused at the closing plates with the entoderm of 

 the second, third and fourth pharyngeal pouches. 





Branchial 

 cleft 1 



Cervical sinus 



Cervical duct 

 Thymus anlaqe 



Epithelial body of 

 4 n pouch 



Trachea 



Pharyngeal 

 pouch 1 



Pharyngeal 

 pouch Z 



Pharyngeal 

 pouch 3 



Pharyngeal 

 pouch 4~ 



■Apical bud of ' riaht Lung 



Siomach 



Dorsal 'pancrea 



Gall bladder 



Duodenum 



Fig. 162. — A reconstruction of the pharynx and fore-gut of an n.8 mm. embryo seen in dorsal view (after 

 Hammar). The ectodermal structures are stippled. 



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

 to an entirely different type of permanent structures. With the broadening of 

 the pharynx the first two pouches acquire a common opening into it, the primary 

 tympanic cavity. The first pouch later differentiates into the tympanic cavity 

 of the middle ear and into the Eustachian tube. By the growth and lateral 

 expansion of the pharynx the second pouch is taken up into the pharyngeal wall, 

 its dorsal angle alone persisting to be later transformed into the palatine tonsil. 



