750 J. F. GUDERNATSCH 



development of the basi- and hypobranchialia and occur only 

 laterally and ventrally of the aorta. Further back even the lat- 

 eral follicles disappear and only a few small ventral ones are 

 grouped around a small vessel. Space again becomes available 

 towards the second branchial arteries, since the skeletal parts 

 retract more and more, and the follicles reappear in their former 

 locations. The lateral extension however is not as great as in 

 the region of the first arteries, since longitudinal muscle bundles 

 prevent it (fig. 5, B). Dorsally the follicles again reach up to 

 the pharyngeal floor. Close to the second branchial arteries 

 the dorsal extension again diminishes and almost disappears 

 when the second gill arteries are reached. Here the follicles lie far 

 below the aorta, as they are forced away from the vessel by a 

 longitudinal muscle. From this place backward a few follicles 

 again appear above the aorta; they are small and scarce, five or 

 six in a section, and widely scattered. Behind the third arteries 

 the ventral aorta lies buried far beneath a muscle, between which 

 and the skeletal parts a portion of the thyreoid lies. Another 

 portion lies below the aorta between the third and fourth aortic 

 branches, and here once more the amount of thyreoid tissue is 

 slightly increased. A small mass of follicles disconnected from 

 the main mass appears behind this place, lying below the aorta. 



Cross sections through the follicles are usually circular, some 

 are irregular. Their size decreases from the anterior towards 

 the posterior end of the thyreoid region. The diameters vary 

 between 10 and 60^ in a single section. The epithelium is rather 

 flat, though some follicles have a cubical epithelium 3 to 5m 

 high. The nuclei of the flat cells show a peculiar feature; in all 

 other cases they are either round or oval, but here with a few 

 exceptions they are bent, taking forms ranging from wide arches 

 to perfect horse-shoe shapes and are from 8 to 10/x long. In all 

 probability they are degenerating, since they do not stain as 

 deeply with nuclear dyes as do the round nuclei. Many of the 

 follicles do not contain colloid. 



The blood supply of the thyreoid zone is rich but there are no 

 capillaries to the follicles proper, although there are smaller 

 blood vessels in the region. Large veins and lymph vessels lie 

 around the aorta and the follicles lie close to their walls (fig. 5, B) . 



