THYBEOID GLAND OF THE TELEOSTS 



753 



Fig. 6. A. Section through the thyreoid gland of a one month old specimen of 

 Salvelinus fontinalis, between the first and second branchial arteries. 



B to D. Sections through the thyreoid gland of an adult specimen. B and C, 

 in the aortic bifurcation; D, near the second branchial arteries. 



the aorta a dense arrangement of follicles is seen and below the 

 number increases. Laterally the follicles extend along the arter- 

 ies. Behind this place the aortic stem is entirely surrounded by 

 thyreoid tissue which completely fills the space between bones 

 and muscles. Small follicles are imbedded in the adventitia of 

 the aorta. There is a dissolution of the dense arrangement in 

 the peripheral zones, especially ventrally. Close behind the 

 second branchial arteries the mass of thyreoid tissue decreases 

 very suddenly and only a thin ring of follicles surrounds the aorta. 

 Towards the third branchial arteries the aorta becomes buried 

 between the ventral muscles, and the ventral follicles disappear 

 sooner than the dorsal ones which continue and surround the 

 third branchial arches for a short distance. Behind the third 



