THYREOID GLAND OF THE TELEOSTS 747 



are not visible. Around the aorta there are rather large veins 

 or lymph vessels with extremely thin walls, close to which the 

 follicles lie (fig. 4, B). There is no tissue (basement membrane) 

 between the epi- and endothelium, the first being almost as thin 

 as the latter. The nuclei of these epithelial cells are spindle- 

 shaped and lie far apart. 



Specimen one year old. In this fish the distribution of the thy- 

 reoid is about the same as in the younger one, (pi. II, fig. 23). 

 The anterior end is pushed further forward in the aortic bifurca- 

 tion, and the posterior end still lies close to the third gill branches. 

 The mass of thyreoid tissue is much enlarged. The follicles are 

 much larger in the bifurcation, and in a section there are more 

 than three times as many as in a one month old individual. They 

 are packed more densely and completely fill the space between 

 the cartilages and arteries. The process of the copula mentioned 

 above, which comes down to the level of the aorta, here divides 

 the thyreoid into a right and left half. While in the younger 

 trout the lateral extension of the follicles was less than the dorso- 

 ventral, at this age the floor of the pharynx has become broader 

 through a widening out of the gill arches, and the lateral distri- 

 bution is more than twice as extensive as the dorso-ventral, 

 although the follicles still go high up along the cartilages (fig. 4, 

 C). The follicles also extend some distance along the first 

 branchial arteries. Here the entire thyreoid lies dorsal to the 

 blood vessel and is grouped around two or more large lymph 

 spaces (fig. 4, C). Immediately behind the aortic bifurcation 

 the lateral and then the dorso-ventral dispersion of the follicles 

 decrease, so that they lie more densely packed and are fewer in 

 number. 



The hypobranchialia approach closer and closer to the copula 

 as we pass backward and force the thyreoid to a more ventral 

 position. Finally the aorta lies almost on the cartilages and the 

 thyreoid shows only one or two follicles in the section. This 

 restriction of the thyreoid zone (pi. II, fig. 23) between the first 

 and second branchial arteries is typical for all salmonids. It may 

 also occur in some other species but is never so pronounced as in 

 the trout. 



