ni THYROID 175 



DERIVATIVES OF PHARYNGEAL WALL OTHER THAN THE 

 RESPIRATORY ORGANS 



Thyroid. — The Thyroid gland arises as a mid-ventral outgrowth 

 of the pharyngeal or buccal floor about the level of the Hyoid 



Fig. 99. — Sagittal sections through anterior portion of alimentary canal of Lepidosiren 

 illustrating the development of the Thyroid. 



A, B,, C from specimens of stage 30 ; D, stage 31 ; Th, thyroid ; t, tongue. 



arch. In those. Vertebrates in which the pharyngeal rudiment is solid 

 at this stage the thyroid outgrowth is also solid at its first appearance 

 (Fig. 99, A, Th) and develops its cavity secondarily by cytolysis. 



