262 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



of the neck. The process of obliteration of these external grooves 

 is interesting. The anterior arches, especially the hyoid, enlarge 

 and slide back over those more posterior, so that at least the external 

 branchial grooves lie in a pocket, the cervical sinus (figs. 282, 286). 

 Later a process from the hyoid arch extends over and closes the sinus, 

 a process recalling the history in the anura. Internally the ento- 

 dermal branchial pouches, with the exception of the first, disappear, 

 but the first persists as the tympanic cavity and the Eustachian tube 

 described in connexion with the ear (p. 202). 



PHARYNGEAL DERIVATIVES 



Several structures arising in the pharyngeal region, some from gill 

 clefts, some from other parts of the walls of the pharynx, naturally 

 comes for mention here, though none of them are respiratory in 

 character. 



Fig. 283. — Schemes of the origin of several pharyngeal derivatives in (^) Rata, (B) 

 anuran and (C) chick, after Verdun, cd, carotid gland; e, epithelial body; gr, gill-rem- 

 nants; p, postbranchial body; tm, thymus; tr, thyreoid; I-VI, giU pouches or clefts. 



Among these are the thymus glands, the functions of which are 

 as yet very obscure. They arise as solid outgrowths from the dorsal 

 angle (the ventral angle in mammals) of a varying number of gill 

 pouches. Usually they are regarded as entodermal, but a few ob- 

 servers claim that there is an ectodermal contribution to them. 



In the cyclostomes apparently all seven (lampreys) clefts are concerned 

 in their development. In the elasmobranchs (fig. 283, A) clefts 2-6 and 

 possibly the spiracle contribute; in some instances the sixth does not. In 

 teleosts and caecilians the numbers are 2-6; in urodeles 1-5, the first and second 

 degenerating. In the anura (fig. 283, B) permanent contributions come from 



