246 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



fourth in teleostomes), and it maintains its branchiomeric character in 

 snakes and gymnophiones. It may lie above and behind the angle of the 

 jaw (most amphibians), close to the carotid arteries (most sauropsida), 

 sometimes extending along the neck (crocodiles and birds). In the 

 young mammals the thymus (sold in the markets as 'throat sweet- 

 breads'), which arises from a single pair of clefts, is largely behind the 

 sternum, extending forward along the neck. Later it gradually grows 

 smaller, the extreme development being reached in man between the 

 fourteenth and sixteenth years, but retaining its functional structure 



FIG. 254. Schemes of the origin of several pharyngeal derivatives in (A} Raia, (B) 

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

 p, postbranchial body; tm, thymus; tr, thyreoid; 7-77, gill pouches or clefts. 



until middle life. The function of the thymus glands is as yet unknown ; 

 though leucocytes are abundant in them, they are not lymphoidal in 

 character. 



Other structures arising in the pharynx, either from the gill clefts 

 or from the pharyngeal walls, are the ' epithelial bodies,' post-branchial 

 bodies, suprapericardial bodies, gill remnants, etc., concerning which 

 little is known. The carotid glands of the same region are referred to 

 elsewhere. 



The thyreoid gland cannot be dismissed in such a summary 

 manner. This is a ductless gland in the pharyngeal region of all 

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

 it arises as an unpaired pocket in the floor of the pharynx (fig. 254), 

 this retaining its connexion with the parent tube in the ammocoete 

 stage of the lamprey (fig. 190), but at the time of metamorphosis it 

 loses its duct (as is early the case in all other vertebrates) and eventu- 



