222 



THE BIOLOGY OF THE FROG 



CHAP. 



A' R 



Fig. 6o. — Diagram show- 

 ing the position of the 

 thyroid glands, t; I, 

 lateral process of hyoid 

 cartilage ; /./, thyro- 

 hyoid process of hyoid. 



tion in Rana pipiens. 



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- 

 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 Hned by a sin- 

 gle layer of cubical 



epithelial cells. In 



the center of each 



follicle is a colloidal 



mass of transparent 



substance which 



probably represents the secretion of the epithelial lining. 

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



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



e-" 



Fig, 6i. — Part of a cross section of the thy- 

 roid of Rana pipiens. e, epithelial layer of 

 vesicles ; m, colloidal substance in vesicle. 



