220 Basic Structure of Vertebrates 



localization in the body. In fishes the pharyngeal glands are small and 

 numerous. Many or all of the pharyngeal pouches may produce glands. 

 In amphibians and reptiles, and culminating in birds and mammals, 

 there appears a tendency toward reduction in number of glandular 

 outgrowths and further reduction in ultimate number by fusion of 

 embryonic units. The persisting glands in birds and mammals are, 

 for the most part, in the more posterior pharyngeal region. 



Thyroid Gland 



The thyroid gland, present in all vertebrates, arises as a mid- 

 ventral outgrowth from the pharynx opposite the first pair of pharyn- 

 geal pouches (Fig. 187). It becomes completely detached from the 

 floor of the pharynx and remains near its place of origin or may shift 

 to a somewhat more posterior position. In terrestrial vertebrates it 



POUCH 4: 



HYPOPHYSIS 



•MOUTH 

 _ 7 P0UCH 1 . 



UTeustachian tube) 

 -thyroid gland 

 -palatine tonsil 



pr<- "POUCH 2 



V^gL -PARATHYROID 1 



■ POUCH 3 



Ks-THYMUS 1 

 ^-PARATHYROID 2 

 ^""THYMUS 2 

 POSTBRANCHIAL BODY 



%,-LUNG LOBE 



ESOPHAGUS 



Fig. 187. Ventral view of pharyngeal region of a human embryo showing the 

 pharyngeal pouches and their glandular derivatives; semidiagrammatic. (Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



