246 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
fourth in teleostomes), and it maintains its branchiomeric character in 
snakes and gymnophiones. It maylieaboveand behind theangle 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 
Fic. 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; ¢m, thymus; #7, thyreoid; I-VI, 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 ammoccete 
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- 
