THYREOID GLAND OF THE TELEOSTS 761 



The formation of colloid ordinarily occurs in only a part of the 

 thyreoid at a time. Here, however, the entire gland seemed to be 

 in a similar physiological state. 



FUNDULUS HETEROCLITUS L. 



Specimens 10 cm. in length. The follicles in the region of the 

 aortic bifurcation are grouped around a vein, most of them lying 

 to the sides of it and under a transverse muscle. The elliptical 

 shape of the vein in sections indicates the pressure between this 

 muscle and the m. sternohyoideus which forces the follicles out 

 from the median line. The follicles become more numerous to- 

 wards the aortic bifurcation and they extend part way out along 

 the first branchial arteries, and more on their ventral than dorsal 

 side. Between the first and second gill branches follicles are found 

 under and above the transverse muscles around which they have 

 traveled. The ventral aorta in this region is completely sur- 

 rounded by thyreoid tissue, more being found on the sides than 

 either dorsally or ventrally (fig. 10, B). At the second gill branches 

 the follicles again spread out laterally. Behind this place only 

 a few scattered follicles are found (pi. I, fig. 6). 



The size of the follicles varies extremely. The smallest are found 

 at the anterior end and the largest in the middle of the thyreoid 

 region. They are either circular in cross section, oval or with 

 irregular evaginations. 



The epithelial cells are usually cubical, but in very small folli- 

 cles sometimes columnar, while in large empty follicles the cells 

 are flat. Narrower cells with spindle shaped nuclei are seen in 

 places. 



The colloid is granular, and in some regions is seen to leave the 

 follicle. Whether this is due to artificial pressure cannot be stated. 

 Occasionally two neighboring epithelial cells will flatten out 

 somewhat as if they were about to form a passage between them. 



The blood supply to the thyreoid region is rich. The follicles 

 are almost completely surrounded by a net of capillaries. These 

 vessels are so pressed against the follicle that they form grooves 

 in it (pi. IV, fig. 6). The projections of the epithelium between 



