THYREOID GLAND OF THE TELEOSTS 769 



even here there are only ten or fifteen follicles in a cross section. 

 Some follicles lie close to the base of the second gill arteries and 

 from this point the gland extends, with from six to ten follicles 

 in a cross section, to a little behind the third branchial arteries 

 where it ends. The follicles are generally dorsal to the ventral 

 aorta (fig. 13, B), only a few being below it. 



The form of the follicles is irregular, but approaches the globu- 

 lar type. Their size ranges from 15-100/x in diameter though 

 some are far above this size (giant follicles). The minute histology 

 shows no peculiarities. The epithelium is usually cubical, the 

 cells being 6ai high. 



MORONE AMERICANA GMELIN 



Specimen 35 cm. in length. The thyreoid gland of the white 

 perch is characterized by the enormous size of nearly all the 

 follicles as well as by their unusually loose arrangement. Cepha- 

 lad of the aortic bifurcation there is little room for dispersion 

 since the copula reaches far down and the skeletal arch is rather 

 narrow. Behind the bifurcation (fig. 14, B) this arch becomes 

 wider and from here to the second gill arteries the main mass of 

 the thyreoid is situated (pi. I, fig. 12). From the second branchial 

 arch towards the third two narrow lines of follicles run along the 

 sides of the aorta. The entire length of the thyreoid region meas- 

 ures 3.5 cm. The majority of the follicles lie above the aorta ex- 

 cept in the anterior region. 



The size of the follicles varies from 120 to 600ai in diameter, 

 the very large ones are most abundant especially in the more 

 anterior region. In cross sections the follicles are almost all circu- 

 lar. The epithelial cells are low, 3 to 4^ high. In these follicles 

 there are no indentations in the colloid, it either fills out the lumen 

 completely or is retracted from the epithehum and has a smooth 

 edge. (The differences in the colloid of different species may be 

 of some physiological significance.) 



