752 J. F. GUDERNATSCH 



situated above the aorta; near the second a few appear below the 

 aorta (fig. 6, .4). 



Length 25 to 30 cm. In the brook trout a condition of remark- 

 ably wide distribution of thyreoid material is seen. The region 

 of the thyreoid in this species is comparatively larger than in 

 any other fish. The anterior end of the gland is far in front of 

 the aortic bifurcation and small follicles extend to the floor of 

 the pharynx (fig. 6, B). 



The first branchial arches are completely surrounded by thy- 

 reoid follicles. In the aortic bifurcation the follicles are very 

 numerous, densely packed and occupy a rather large field. They 

 reach up to the dorsal edge of the copula and laterally to the gill 

 bases. On both sides of the aorta they are scattered between 

 the fibres of longitudinal muscles (fig. 6, C). The follicles force 

 their way through ^the muscle tissue along blood vessels and con- 

 nective tissue fibres. Below the aorta their arrangement is less 

 dense. Close behind the aortic bifurcation the amount of thy- 

 reoid tissue is reduced in the typical way, the copula extending 

 down to the aorta. By this arrangement three, more or less 

 separated, thyreoid masses are formed, two dorsally to the right 

 and left of the copula and one below the aorta. The ventral part 

 decreases, then the dorsal masses, the arrangement of the fol- 

 licles becoming looser. Although the dorsal space becomes more 

 open the follicles still decrease in size and are scattered far apart, 

 indicating that this is a zone between two accumulations of thy- 

 reoid tissue, those around the first and second aortic branches. 

 Two centers of growth may easily be determined. 



Just before reaching the second branchial arteries the lateral 

 extension becomes very great (fig. 6, D). The follicles migrate 

 into the first gill arches along the branchial arteries and occur 

 at the base of and extend into the second gill arches. This wide 

 distribution of thyreoid elements is certainly the most remark- 

 able feature of the organ in the Teleosts. Follicles not only lie 

 at the base of the gills, but are distributed along the laminae at 

 the base of the villi. 



At the second branchial arteries the thyreoid gland, as men- 

 tioned above, once more shows an extensive development. Above 



