222 



THE BIOLOGY OF THE FROG 



CHAP. 



FIG. 60. Diagram show- 

 ing the position of the 

 thyroid glands, /; /, 

 lateral process of hyoid 

 cartilage ; t.p, thyro- 

 hyoid process of hyoid. 



convert the proteid- splitting enzyme of that organ into an 

 active form. Doubt has, however, been recently thrown upon 



this conclusion. 



The Thyroid Glands. The thy- 

 roid glands of the frog are com- 

 pletely separated from each other, 

 being situated on either side of the 

 hyoid apparatus in a small space 

 between its posterior lateral and 

 thyro-hyoid processes. Gaupp has 

 described some thyroid tissue (ac- 

 cessory thyroid) on the ventral side 

 of the hyoglossus muscle, and I have 

 been able to confirm this observa- 

 tion in Rana pipiens. The tissue of the thyroid shows a 

 unique structure, being composed of a mass of rounded 

 follicles united by a 

 small amount of con- 

 nective tissue in 

 which there is a rich 

 supply of blood ves- 

 sels. Each follicle 

 is a perfectly closed 

 sac lined by a sin- 

 gle layer of cubical 

 epithelial cells. In 

 the center of each 

 follicle is a colloidal 

 mass of transparent 

 substance which 



e--- 



FlG. 61. 1'art of a cross section of the thy- 

 roid of Rana pipiens. e, epithelial layer of 

 vesicles; ;;;, colloidal substance in vesicle. 



probably represents the secretion of the epithelial lining. 



The thyroid of the frog, like that of the higher vertebrates, 

 has been found to secrete a substance rich in iodin (iodo- 



