STRUCTURE OF THE VERTEBRATES 



207 



comes differentiated into a glandular structure. These glands, 

 which will be discussed in Chapter XIX, are ductless and func- 

 tion as glands of internal secretion. The derivatives of the gill 

 pouches are: 



I, the paired Eustachian tubes; 



II, the palatine tonsils, which retain their position in the 

 pharynx throughout life; 



III, the thymus gland, the two halves of which unite and mi- 

 grate posteriorly, the adult gland being located near the bifur- 

 cation of the trachea; 



Thyroid 



A Ventral View 



B. Longitudinal Section 



Fig. 120. Derivatives of the Human Pharj^nx. The ventral view (A) shows 



the branchial pouches, the thyroid and lungs. The diagrammatic horizontal 



section shows the glandular derivatives of pouches I to V. 



Ill and IV, develop two pairs of small glands, the parathy- 

 roids, which lie against the thyroid cartilage of the larynx em- 

 bedded in thyroid tissue ; and 



V, the postbranchial bodies. 



The development of the middle ear and Eustachian tube from 

 the first pouch is a direct change of function of a structure. The 

 pharyngeal glands are derivatives of the pouches, not modifica- 

 tions of the structures themselves. 



2. Internal Gills. Internal gills develop within the gill slits 

 of the early chordates, the fish, and the larvae of most amphibia. 

 In several amphibian groups they are carried over to the adult. 



