232 



INTERNAL SECRETION 



What is believed to be the active principle of the thyroid 

 has now been isolated, and is known as thyroxin. It is a 

 compound of tryptophane and iodine. 



The thyroid appears as an outgrowth of the entoderm, 

 hning the floor of the pharynx between the first and second 



Fig. 31. — Thyroid of wild iv.t sliowing tiattoned cells and vesicles 

 distended with colloid (Chalmers Watson, from Schafer, The 

 Endocrine Organs). 



branchial clefts. At first it forms a sohd column of cells 

 which, opposite the upper end of the trachea, divides into two 

 lateral parts. From these, by a process of budding, the 

 thyroid is formed. The sohd column becomes temporarily 

 canahsed and serves as a duct. After this it disappears, 

 its pharyngeal extremity persisting as the foramen caecum 

 of the tongue (Fig. 30). 



