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. 
Fic. 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); /, lung; /g, lacrimal duct; 2, naris; md, mandible; 
on, oronasal groove; ér, 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 
cecilians, 2-6; urodeles, 1-5, 1 and 2 degenerating; anura, 1 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 
areinterpreted as pronenephric. Insome cases (fishes, etc.) the thymus 
retains its primitive position dorsal to the gill clefts (usually above the 
