THE DIGESTIVE TRACT 



58T 



The Gill Filaments are the branched processes of the gills which 

 become quite vascular, and as they project into the opercular cavity, they 

 permit the exchange of gases from the respiratory current of water en- 

 tering the mouth, which current then passes through the gill clefts and 

 the opercular tube. n 



th _ The Velar Plates and 



Gill Rakers (Fig. 341, 

 filtr) form as tiny folds in 

 the pharyngeal region 

 which can be best under- 

 stood by a study of the 

 cuts. (The velar plates 

 are the smooth inturnings 

 of the floor of the pharynx 

 while the jagged tooth- 

 like foldings are the gill 

 rakers.) 



The structures men- 

 tioned above are prac- 

 tically lost when the frog 

 reaches adultship, al- 

 though portions of these 

 structures are used in 

 building and forming 

 other structures which 

 will be discussed shortly. 

 The thymus body 

 (Fig. 342) appears just 

 prior to hatching. It is a 

 solid internal proliferation 

 from the epithelium of the 

 upper side of the first 

 branchial pouch (second 

 visceral, or hyobranchial 

 pouch). There is also a 

 similar outgrowth from the hyomandibular pouch, but this disappears in 

 a short time. The outgrowth which is to become the thymus grows 

 slowly and separates entirely from the wall of the pouch when the tad- 

 pole is about twelve millimeters in length. 



Both permanent and transient thymus bodies lie close to the outer 

 surface of the head, immediately behind the auditory capsule and the 

 articulation of the jaw. 



Epithelioid bodies are similar to the thymus glands. These are 

 formed from the dorsal ends of the other branchial clefts and become 

 lymphoid in character. 



D 



Fig. 342. 



I. Diagram of the branchial pouch derivatives in the 

 frog. (After Maurer, with Greil's modification.) eg. Carotid 

 gland ; e v e^, e 3 , epithelioid bodies ; th, thyroid body ; 

 tm lt tm y , thymus bodies ; ub, ultimobranchial body ; I-V1, 

 first to sixth visceral pouches. (7, hyomandibular ; II-V1, 

 first to fifth branchial pouches.) 



II. Diagrams of the derivatives of the visceral pouches 

 and arches in the frog. A. Lateral view, frog larva. B. 

 Lateral view, after metamorphosis. C. Transverse section 

 through gill of frog larva. D. Transverse section through 

 gill region, just after metamorphosis ; the gills have not 

 quite disappeared, a, Afferent branchial arteries ; c, carotid 

 gland ; d, dorsal gill remainder ; e, epithelioid bodies ; g, 

 internal gills ; m, middle gill remainder ; o, operculum ; , 

 suprapericardial or postbranchial body ; t, thyroid body ; th, 

 thymus bodies ; v, ventral gill remainder ; I-VI, visceral 

 arches ; /, mandibular arch ; II, hyoid arch ; III-VI, first to 

 fourth branchial arches ; 1-6, visceral pouches ; 1, hyo- 

 mandibular pouch ; 2, hyobranchial pouch ; 3-6, first to 

 fourth branchial pouches. (Greil's Modification of Maurer.) 



