THYREOID GLAND OF THE TELEOSTS 757 



In front of the aortic bifurcation the basi- and hypobranchia- 

 lia reach down and here only a few follicles are found on both 

 sides of the hypohyalia. Back of the place where the cartilages 

 have retracted the longitudinal muscle bundles prevent the lat- 

 eral expansion of the thyreoid (pi. V, fig. 14). At the second 

 branchial arteries however, the mass of thyreoid tissue is very 

 much increased, again surrounding the aorta (fig. 7, B). The 

 ventral extension 'is pronounced. As a rule in the trouts no fol- 

 licles lie directly against the aortic wall but here there is a com- 

 plete ring of them around it. Above this ring lies a large lymph 

 sinus and between it and the skeletal parts thyreoid tissue is 

 again found. Towards the third aortic branches the cartilages 

 again compress the aorta, and here the follicles lie around the 

 aorta and along the outlines of the cartilages. Further back 

 the aorta sinks down between the muscles, the ventral follicles 

 disappear and the dorsal ones do not follow the vessel, but increase 

 in number and group themselves around a subcopula between 

 the third branchial branches. This dorsal rather compact group 

 of follicles extends back behind the fourth branchial arteries. 

 Below the aorta a small cluster of four or five follicles appears 

 as is seen in other species of trout (fig. 7, C) . At the level of the 

 fourth aortic branches the copula extends so far ventrally that 

 the dorsal follicles are pressed between the muscles and again 

 come down into contact with the aorta. The posterior end of 

 the thyreoid is in this region of the fourth arch. 



The form of the follicles is usually elliptical, though circular 

 cross sections are also found, ranging from 15 to lOO/x in diame- 

 ter. The larger ones are more abundant. 



The follicular epithelium is very flat (pi. V, fig. 14), and in 

 most of the cells are again seen the irregular nuclei described in 

 some of the above species of trout (pi. IV, fig. 9, N). The major- 

 ity of follicles are in close relation with large lymph sinuses, epi- 

 and endothelium being in contact (fig. 7, B, C). There are no 

 capillaries to the follicles proper. 



JOURNAL OF MORPHOLOGY, VOL. 21, NO. 4 



