RESPIRATORY ORGANS. 245 



calling the history in the anura. Internally the entodermal branchial 

 pouches, with the exception of the first, disappear, but the first persists 

 as the tympanic cavity and Eustachian tube described in connexion 

 with the ear. 



FIG. 253. Head of human embryo with pharyngeal floor removed, after Hertwig. 

 Cut surfaces lined. Compare with fig. 221. cs, cervical sinus; e, eye; h, hyoid arch; hd, 

 hypophysial duct (Rathke's pocket); 7, lung; Ig, lacrimal duct; n, naris; ntd, mandible; 

 on, oronasal groove; tr, trachea. 



Pharyngeal Derivatives. 



Several structures arise in the pharyngeal region some developed 

 from gill clefts, some from other parts which, while not respiratory 

 in character, naturally come for mention here. 



Among these are the thymus glands. These arise from the ento- 

 dermal epithelium at the dorsal angle of a varying number of visceral 

 clefts (elasmobranchs, clefts 2-6 and possibly the spiracle; teleosts and 

 caecilians, 2-6; urodeles, 1-5, i and 2 degenerating; anura, i and 2, 

 the latter only persisting; amniotes 3 and 4). 



The organ which results has varying positions and shapes in the 

 different groups. It becomes richly vascular, and by the intrusion of 

 connective tissue, assumes an acinous form. In Myxine a number of 

 lobules behind the gill region have been regarded as a thymus, but now 

 are interpreted as pronenephric. In some cases (fishes, etc.) the thymus 

 retains its primitive position dorsal to the gill clefts (usually above the 



