PHARYNGEAL DERIVATIVES 



265 



ing to some authors the second gill pouch in reptiles, the third in amphibia, 

 contributes to them, but this is denied by others. In the birds and mammals 

 they are described as products of the vascular walls, with no epithelial contribu- 

 tions. Of their function in the amniotes nothing is known. In the adult am- 

 phibia they lose their plexiform character and form a muscular thickening which 

 projects into the vessel, and it has been suggested that this serves as an accessory 

 heart. 



The thyreoid is a ductless gland in the pharyngeal region of all 

 vertebrates, ventral to the alimentary tract. In the lower verte- 

 brates it arises as an unpaired pocket in the floor of the pharynx in 

 the region of the second to fourth gill clefts (fig. 283), and in the lar- 

 val (Ammocoeetes) lamprey this retains its connexion with the 



Fig. 286. — Diagram of the floor of the mouth of an embryo mammal showing the 

 early stages of the tongue and pharyngeal derivatives, cs, cervical sinus; fc foramen 

 caecum; A, hyoid cleft; m, mandibular part of tongue; md, mandibular arch; 0, oesophagus; 

 p, aniage of parathyreoid; /, anlage of tonsil; ti, tuberculum impar; Im, tympanic mem- 

 brane; tms, anlage of thymus; ty, thyreoid anlage; 1-4, branchial clefts. 



pharynx (fig. 211) until metamorphosis when it loses its duct and 

 eventually becomes a complex of follicles. In other vertebrates it 

 arises as a pocket between the first and second clefts and lies primi- 

 tively just in front of the pericardium. It takes the form of a 

 vesicle which sends out solid cords which later become follicular. 



Usually the thyreoid is markedly bilateral, and in amphibia and 

 birds (fig. 284) it divides into two glands. In the elasmobranchs it 

 lies between the anterior end of the ventral aorta and the symphysis 

 of the lower jaw. In teleosts the paired groups of follicles lie around 

 the ventral aorta, extending laterally along the anterior aortic arches. 

 The gland lies just behind the second arch in urodeles, and in anura 



