THYREOID GLAND OF THE TELEOSTS 773 



a globular shape. This indicates that the shape of the follicles 

 is a result of the pressure directions. The follicular arrangement 

 is rather compact in the central portions, (pi. V, fig. 17.) 



The epithelial cells vary from low cuboidal to high cylindrical 

 shapes. The smaller follicles seem to have a little higher epithe- 

 lium, though it is rather uniform in the same individual and varies 

 more among the several specimens. It may seem therefore that 

 the entire gland is in the same physiological stage. 



MICROPOGON UNDULATUS L. 



A croaker, 30 cm. long. The thyreoid extends from the first to the 

 third branchial arteries (pi. II, fig. 15). The dispersion of folli- 

 cles is largely dorso-ventral, since laterally they are hindered by 

 the narrowness of the isthmus (fig. 16, A, C). For this reason also 

 a considerable part of the gland lies below the aorta, yet not so 

 large a portion as above, though the dorsal follicles are less densely 

 arranged. 



There are only a few follicles in front of the aortic bifurcation, 

 yet at the bifurcation and behind it lies the main mass of the gland. 

 The follicles completely fill the spaces between bones and vessels 

 (fig. 16, A). Towards the second gill branches the copula extends 

 further and further down and forces the follicles into a somewhat 

 lateral position. The ventral mass is larger in this region. At 

 the second arterial branches there is no special increase in mass, 

 the number of ventral follicles having decreased (fig. 16, B), the 

 dorsal ones increasing and soon extending to the epithelium of 

 the pharyngeal floor. The follicles lie rather loosely arranged, but 

 have not noticeably increased in size. A small line of follicles 

 above the aorta extends from here towards the third gill branches, 

 others are scattered irregularly around the aorta. The aorta has 

 sunk into the ventral muscle and carries the posterior folhcles 

 with it. 



The thyreoid gland of Micropogon is characterized by rather 

 small follicles of almost uniform size, though in some regions 

 large ones appear. The diameters range from 10 to 300m, but 

 those of 30 to 50/x are most abundant. 



JOURNAL OF MORPHOLOGY, VOL. 21, NO. 4 



